Underwriting Agreement

Exhibit 1.1

EXECUTION VERSION

ALLIANT ENERGY CORPORATION

(a Wisconsin corporation)

4.00% SENIOR NOTES DUE 2014

UNDERWRITING AGREEMENT

Dated: September 30, 2009

TABLE OF CONTENTS

i

ii

iii

SCHEDULES

Schedule A – List of Underwriters’ Purchase Amounts

Schedule B – Issuer Free Writing Prospectuses contained in the General Disclosure Package

Schedule C – Pricing Information

Schedule D – Final Term Sheet

iv

ALLIANT ENERGY CORPORATION

(a Wisconsin corporation)

4.00% SENIOR NOTES DUE 2014

UNDERWRITING AGREEMENT

September 30, 2009

Citigroup Global Markets Inc.

388/390 Greenwich Street

New York, NY 10013

J.P. Morgan Securities Inc.

270 Park Avenue

New York, NY 10017

Ladies and Gentlemen:

Alliant Energy Corporation, a Wisconsin corporation (the “ Company ”), confirms its agreement with Citigroup Global Markets, Inc. (“ Citi ”) and J.P. Morgan Securities Inc. (“ J.P. Morgan ” and, together with Citi, the “ Underwriters ”, which term shall also include any underwriter substituted as hereinafter provided in Section 10 hereof), with respect to the issue and sale by the Company and the purchase by the Underwriters, acting severally and not jointly, of the respective principal amounts set forth on Schedule A of $250,000,000 aggregate principal amount of the Company’s 4.00% Senior Notes due 2014 (the “ Securities ”). The Securities will be issued pursuant to an indenture dated as of September 30, 2009 (the “ Base Indenture ”) between the Company and Wells Fargo Bank, National Association, as trustee (the “ Trustee ”). The term “ Indenture ,” as used herein, includes the Base Indenture and the Officer’s Certificate (as defined in the Base Indenture) to be executed in connection with the offering of the Securities, establishing the form and terms of the Securities pursuant to Section 301 of the Base Indenture. The Securities are to be issued in book-entry form and will be registered in the name of Cede & Co. as nominee of The Depository Trust Company (“ DTC ”) pursuant to the blanket letter agreement, dated September 25, 2009 (the “ DTC Agreement ”), between the Company and DTC.

The Company understands that the Underwriters propose to make a public offering of the Securities as soon as the Underwriters deem advisable after this Agreement has been executed and delivered.

The Company has filed with the Securities and Exchange Commission (the “ Commission ”) an “automatic shelf registration statement” as defined under Rule 405 (“ Rule 405 ”) of the Securities Act of 1933, as amended (the “ 1933 Act ”) on Form S-3 (No. 333-162214),

1

including a base prospectus (the “ Base Prospectus ”), for the registration of the Securities under the 1933 Act and the offering thereof from time to time in accordance with Rule 415 of the rules and regulations of the Commission under the 1933 Act (the “ 1933 Act Regulations ”), which registration statement was filed with the Commission not earlier than three years prior to the date hereof and became effective on filing with the Commission under Rule 462(e) of the 1933 Act Regulations (“ Rule 462(e)” ). Promptly after execution and delivery of this Agreement, the Company will prepare and file a prospectus supplement in accordance with the provisions of Rule 430B (“ Rule 430B ”) of the 1933 Act Regulations and paragraph (b) of Rule 424(b) (“ Rule 424(b) ”) of the 1933 Act Regulations. Any information included in such prospectus supplement that was omitted from the Registration Statement at the time it first became effective but that is deemed to be a part of and included in such registration statement pursuant to Rule 430B is referred to as “ Rule 430B Information .” Such registration statement, at any given time, including the amendments thereto to such time, the exhibits and schedules thereto, at such time, is referred to herein as the “ Registration Statement ;” and the Registration Statement at the time it originally became effective is herein called the “ Original Registration Statement .” The final prospectus supplement, together with the Base Prospectus, in the form first furnished to the Underwriters by the Company for use in connection with the offering of the Securities, is referred to herein as the “ Prospectus .” A “ preliminary prospectus ” shall be deemed to refer to any preliminary prospectus supplement, together with the Base Prospectus, used in connection with the offering of the Securities that omitted the Rule 430B Information. For the purposes of this Agreement, all references to the “Registration Statement,” the “Prospectus” or any preliminary prospectus shall also be deemed to include all documents incorporated therein by reference pursuant to Item 12 of Form S-3 under the 1933 Act at such time and the documents otherwise deemed to be a part thereof or included therein by 1933 Act Regulations. For purposes of this Agreement, all references to the Registration Statement, Prospectus or preliminary prospectus or to any amendment or supplement to any of the foregoing shall be deemed to include any copy filed with the Commission pursuant to its Electronic Data Gathering, Analysis and Retrieval system (“ EDGAR ”).

All references in this Agreement to financial statements and schedules and other information which are “contained,” “included” or “stated” (or other references of like import) in the Registration Statement, Prospectus or preliminary prospectus shall be deemed to mean and include all such financial statements and schedules and other information which are incorporated by reference in or otherwise deemed by 1933 Act Regulations to be a part of or included in, as of such applicable date, the Registration Statement, Prospectus or preliminary prospectus, as the case may be; and all references in this Agreement to the Registration Statement, amendments or supplements to the Registration Statement, Prospectus or preliminary prospectus shall be deemed to include the filing of any document under the Securities Exchange Act of 1934, as amended (the “ 1934 Act ”), which is incorporated by reference in or otherwise deemed by 1933 Act Regulations to be a part of or included in, as of such applicable date, the Registration Statement, Prospectus or preliminary prospectus, as the case may be.

SECTION 1. Representations and Warranties .

2

(i) Status as a Well-Known Seasoned Issuer . (A) At the time of filing the Original Registration Statement, (B) at the time of the most recent amendment thereto for the purposes of complying with Section 10(a)(3) of the 1933 Act (whether such amendment was by post-effective amendment, incorporated report filed pursuant to Section 13 or 15(d) of the 1934 Act or form of prospectus), (C) at the time the Company or any person acting on its behalf (within the meaning, for this clause only, of Rule 163(c) of the 1933 Act Regulations) made any offer relating to the Securities in reliance on the exemption of Rule 163 of the 1933 Act Regulations (“ Rule 163 ”) and (D) at the date hereof, the Company was and is a “well-known seasoned issuer” as defined in Rule 405, including not having been and not being an “ineligible issuer” as defined in Rule 405. The Registration Statement is an “automatic shelf registration statement,” as defined in Rule 405, and the Securities, since their registration on the Registration Statement, have been and remain eligible for registration by the Company on a Rule 405 “automatic shelf registration statement”. The Company has not received from the Commission any notice pursuant to Rule 401(g)(2) of the 1933 Act Regulations objecting to the use of the automatic shelf registration statement form.

(ii) Registration Statement, Prospectus and Disclosure at Time of Sale . The Original Registration Statement became effective upon filing under Rule 462(e) on September 30, 2009, and any post-effective amendment thereto also became effective upon filing under Rule 462(e). No stop order suspending the effectiveness of the Registration Statement has been issued under the 1933 Act and no proceedings for that purpose have been instituted or are pending or, to the knowledge of the Company, are contemplated by the Commission, and any request on the part of the Commission for additional information has been complied with.

Any offer that is a written communication relating to the Securities made prior to the filing of the Original Registration Statement by the Company or any person acting on its behalf (within the meaning, for this paragraph only, of Rule 163(c) of the 1933 Act Regulations) has been filed with the Commission in accordance with the exemption provided by Rule 163 and otherwise complied with the requirements of Rule 163, including without limitation the legending requirement, to qualify such offer for the exemption from Section 5(c) of the 1933 Act provided by Rule 163.

At the time the Registration Statement became effective (including without limitation any effective dates of any amendments thereto and each deemed effective date with respect to the Underwriters pursuant to Rule 430B(f)(2) of the 1933 Act Regulations) and at the Closing Time, the Registration Statement and any amendments and supplements thereto complied or will comply, as the case may be, in all material respects with the requirements of the 1933 Act and the 1933 Act Regulations and the Trust Indenture Act of 1939, as amended (the “ 1939 Act ”) and the rules and regulations of the Commission under the 1939 Act (the “ 1939 Act Regulations ”), and did not or will not, as the case may be, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not

3

misleading and the Indenture complied and will comply in all material respects with the requirements of the 1939 Act. Neither the Prospectus nor any amendments or supplements thereto, at the time the Prospectus or any such amendment or supplement was issued and at the Closing Time, included or will include an untrue statement of a material fact or omitted or will omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading. The representations and warranties in this subsection shall not apply to statements in or omissions from the Registration Statement or Prospectus made in reliance upon and in conformity with information furnished to the Company in writing by any Underwriter through the Underwriters expressly for use in the Registration Statement (or any amendment thereto) or Prospectus (or any amendment thereto).

Each preliminary prospectus and the prospectus filed as part of the Registration Statement as originally filed or as part of any amendment thereto, or filed pursuant to Rule 424 under the 1933 Act, complied when so filed in all material respects with the 1933 Act Regulations and each preliminary prospectus and the Prospectus delivered to the Underwriters for use in connection with this offering was identical to the electronically transmitted copies thereof filed with the Commission pursuant to EDGAR, except to the extent permitted by Regulation S-T.

As of the Applicable Time (as defined below), neither (x) the Issuer Free Writing Prospectus(es) (as defined below) listed on Schedule B and the Statutory Prospectus (as defined below), all considered together (collectively, the “ General Disclosure Package ”), nor (y) any individual Issuer Free Writing Prospectus, when considered together with the General Disclosure Package, will include any untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

As used in this subsection and elsewhere in this Agreement:

“ Applicable Time ” means 1:30 p.m. (Eastern time) on the date of this Agreement.

“ Issuer Free Writing Prospectus ” means any “issuer free writing prospectus,” as defined in Rule 433 of the 1933 Act Regulations (“ Rule 433 ”), relating to the Securities that (i) is required to be filed with the Commission by the Company or (ii) is exempt from filing pursuant to Rule 433(d)(5)(i) because it contains a description of the Securities or of the offering that does not reflect the final terms, in each case in the form filed or required to be filed with the Commission or, if not required to be filed, in the form retained in the Company’s records pursuant to Rule 433(g) or is exempt from filing pursuant to Rule 433(d)(8)(i) relating to road shows.

“ Statutory Prospectus ” as of any time means the prospectus relating to the Securities that is included in the Registration Statement immediately prior to that time, including any document incorporated by reference therein and any preliminary or other prospectus supplement deemed to be a part thereof.

4

Each Issuer Free Writing Prospectus, as of its issue date and at all subsequent times through the completion of the public offer and sale of the Securities or until any earlier date that the issuer notified or notifies the Underwriters as described in the next sentence, did not, does not and will not include any information that conflicted, conflicts or will conflict with the information contained in the Registration Statement or the Prospectus, including any document incorporated by reference therein and any preliminary or other prospectus deemed to be a part thereof that has not been superseded or modified. If at any time following issuance of an Issuer Free Writing Prospectus there occurred or occurs an event or development as a result of which such Issuer Free Writing Prospectus included or would include an untrue statement of a material fact or omitted or would omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances, not misleading, the Company has notified or will notify promptly the Underwriters so that any use of such Issuer Free Writing Prospectus may cease until it is amended or supplemented. The foregoing two sentences do not apply to statements in or omissions from any Issuer Free Writing Prospectus based upon and in conformity with written information furnished to the Company by any of the Underwriters expressly for use therein.

(iii) Incorporated Documents .

(A) The documents incorporated or deemed to be incorporated by reference in the Registration Statement, the Statutory Prospectus and the Prospectus, at the time they were or hereafter are filed with the Commission, complied or will comply, as the case may be, in all material respects with the requirements of the 1934 Act and the rules and regulations of the Commission thereunder (the “ 1934 Act Regulations ”), as applicable, and, when read together with the other information in the Registration Statement, the Statutory Prospectus and the Prospectus, did not or will not, as the case may be, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading.

(B) The description of regulatory matters to which the Company is subject, as disclosed in the Company’s filings with the Commission under the 1934 Act and the 1934 Act Regulations and as incorporated by reference into the Registration Statement, is true and correct in all material respects, except to the extent such description in any specific filing has been superseded, updated or supplemented by such description in a subsequent filing under the 1934 Act or the 1934 Act Regulations made prior to the date hereof and incorporated by reference into the Registration Statement or by such description in the Statutory Prospectus and the Prospectus.

(iv) Independent Accountants . The accountants who certified the financial statements and supporting schedules included in the Registration Statement, the Statutory Prospectus and the Prospectus are independent registered public accountants with respect to the Company and its subsidiaries as required by the 1933 Act and by the rules of the Public Company Accounting Oversight Board.

5

(v) Financial Statements . The financial statements included in the Registration Statement, the Statutory Prospectus and the Prospectus, together with the related schedules and notes, present fairly in all material respects the financial position of the Company and its consolidated subsidiaries at the dates indicated and the statement of income, changes in common equity and cash flows of the Company and its consolidated subsidiaries for the periods specified; said financial statements have been prepared in conformity with generally accepted accounting principles (“ GAAP ”) applied on a consistent basis throughout the periods involved. The supporting schedules included in the Registration Statement, if any, present fairly in all material respects in accordance with GAAP the information required to be stated therein. The selected financial data and the summary financial information included in the Statutory Prospectus and the Prospectus present fairly in all material respects the information shown therein and have been compiled on a basis consistent with that of the audited financial statements included in the Registration Statement.

(vi) No Material Adverse Change in Business . Since the respective dates as of which information is given in the Registration Statement, the General Disclosure Package and the Prospectus, except as otherwise stated therein, (A) there has been no material adverse change in the condition, financial or otherwise, earnings or business affairs of the Company and its subsidiaries considered as one enterprise, whether or not arising in the ordinary course of business nor has there been any developments involving a prospective material adverse change of the Company and its subsidiaries considered as one enterprise, whether or not arising in the ordinary course of business (a “ Material Adverse Effect ”), (B) there have been no transactions entered into by the Company or any of its subsidiaries, other than those in the ordinary course of business, which are material with respect to the Company and its subsidiaries, and (C) except for regular dividends on the common stock, par value $0.01 per share, of the Company (the “ Common Stock ”) in amounts per share that are consistent with past practice there has been no dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock.

(vii) Good Standing of the Company . The Company has been duly organized and is validly existing as a corporation under the laws of the State of Wisconsin and has corporate power and authority to own, lease and operate its properties and to conduct its business as described in the General Disclosure Package and the Prospectus and to enter into and perform its obligations under this Agreement; and the Company is duly qualified as a foreign corporation to transact business and is in good standing in each other jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure so to qualify or to be in good standing would not result in a Material Adverse Effect.

(viii) Good Standing of Subsidiaries . Each “significant subsidiary” of the Company (as such term is defined in Rule 1-02 of Regulation S-X) is listed on Exhibit 21.1 to the Company’s Annual Report on Form 10-K for the year ended December 31, 2008 (each such subsidiary shall be referred to herein as a Significant Subsidiary ” and, collectively, the “ Significant Subsidiaries ”). Each Significant Subsidiary has been duly organized and is validly existing as a corporation or a limited liability company under the laws of the jurisdiction of its formation, has corporate or other power and authority to own, lease and operate its properties and to conduct its business as described in the General

6

Disclosure Package and the Prospectus and is duly qualified to transact business and is in good standing in each foreign jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure so to qualify or to be in good standing would not result in a Material Adverse Effect; except as otherwise disclosed in the Registration Statement, all of the outstanding equity interests of each such Significant Subsidiary has been duly authorized and validly issued and, except for 6,000,000 shares of Interstate Power and Light Company’s (“ IP&L ”) 8.375% Series B Cumulative Preferred Stock, 1,600,000 shares of IP&L’s 7.10% Series C Cumulative Preferred Stock, 29,957 shares of Wisconsin Power and Light Company’s (“ WP&L ”) 4.40% Preferred Stock, 99,970 shares of WP&L’s 4.50% Preferred Stock, 29,947 shares of WP&L’s 4.76% Preferred Stock, 74,912 shares of WP&L’s 4.80% Preferred Stock, 64,979 shares of WP&L’s 4.96% Preferred Stock, 150,000 shares of WP&L’s 6.20% Preferred Stock, and 599,460 shares of WP&L’s 6.50% Preferred Stock, is, and immediately prior to the Closing Time will be, owned by the Company free and clear of any security interest, mortgage, pledge or lien that reasonably would be expected to result in a Material Adverse Effect; none of the outstanding equity interests of any Significant Subsidiary was issued in violation of the preemptive or similar rights of any other holder of equity interests of such Significant Subsidiary that reasonably would be expected to result in a Material Adverse Effect. The only subsidiaries of the Company are (A) the Significant Subsidiaries and (B) certain other subsidiaries which do not constitute a “significant subsidiary” as defined in Rule 1-02 of Regulation S-X.

(ix) Capitalization . The authorized, issued and outstanding capital stock of the Company as of June 30, 2009 are as set forth in the Company’s consolidated condensed balance sheets as included in the Company’s quarterly report on Form 10-Q for the quarter ended June 30, 2009 (except for subsequent issuances, if any, pursuant to existing reservations, agreements or employee benefit plans, or pursuant to the exercise of convertible securities or options outstanding on the date hereof or pursuant to the Company’s Shareowner Direct Plan). All of the issued and outstanding shares of capital stock of the Company have been duly authorized and validly issued and are fully paid and non-assessable and none of the outstanding shares of capital stock of the Company was issued in violation of the preemptive or other similar rights of any securityholder of the Company. The Company is a “holding company” as defined under the Public Utility Holding Company Act of 2005.

(x) Authorization of Agreement . The Company has all requisite corporate power and authority to execute and deliver this Agreement and to perform its obligations hereunder. This Agreement has been duly authorized, executed and delivered by the Company.

(xi) Authorization of the Indenture . The Indenture has been duly authorized, executed and delivered by the Company and duly qualified under the 1939 Act and constitutes a valid and binding agreement of the Company enforceable against the Company in accordance with its terms, except (A) as enforcement thereof may be limited by bankruptcy, insolvency (including without limitation, all laws relating to fraudulent transfers), reorganization, moratorium or similar laws affecting the enforcement of creditors’ rights generally and (B) as enforcement thereof is subject to general principles of equity (regardless of whether enforcement is considered in a proceeding in equity or at law).

7

(xii) Authorization of the Securities . The Securities have been duly authorized and, at the Closing Time, will have been duly executed by the Company and, when authenticated, issued and delivered in the manner provided for in the Indenture and delivered against payment of the purchase price therefor as provided for in this Agreement, will constitute valid and binding obligations of the Company, enforceable against the Company in accordance with their terms, except as the enforcement thereof may be limited by bankruptcy, insolvency (including, without limitation, all laws relating to fraudulent transfers), reorganization, moratorium or similar laws affecting enforcement of creditors’ rights generally and except as enforcement thereof is subject to general principles of equity (regardless of whether enforcement is considered in a proceeding in equity or at law) and will be in the form contemplated by, and entitled to the benefits of, the Indenture.

(xiii) Description of the Securities and the Indenture . The statements relating to the Securities and the Indenture contained in the Prospectus and the General Disclosure Package conform, respectively, in all material respects to the terms of the Securities and the Indenture.

(xiv) Absence of Defaults and Conflicts . Neither the Company nor any of its subsidiaries is in violation of its charter or by-laws or in default in the performance or observance of