SIXTH SUPPLEMENTAL INDENTURE

Exhibit 4.2

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INTERSTATE POWER AND LIGHT COMPANY

(formerly known as IES Utilities Inc.,

formerly known as Iowa Electric Light and Power Company)

TO

J.P. MORGAN TRUST COMPANY, NATIONAL ASSOCIATION

Successor Trustee

__________________________

Sixty-fourth Supplemental

Indenture

Dated as of June 9, 2005

__________________________

SUPPLEMENTAL TO

INDENTURE OF MORTGAGE AND DEED OF TRUST

DATED AS OF AUGUST 1, 1940

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THIS SIXTY-FOURTH SUPPLEMENTAL INDENTURE, dated as of June 9, 2005, between INTERSTATE POWER AND LIGHT COMPANY (formerly known as IES Utilities Inc., formerly known as Iowa Electric Light and Power Company), a corporation organized and existing under the laws of the State of Iowa (hereinafter called the “Company”), and J.P. MORGAN TRUST COMPANY, NATIONAL ASSOCIATION, as successor Trustee, a national banking association organized and existing under the laws of the United States of America.

W I T N E S S E T H:

WHEREAS, the Company has heretofore executed and delivered its Indenture of Mortgage and Deed of Trust, dated as of August 1, 1940 (hereinafter called the “Original Indenture”), to the Trustee to secure the first mortgage bonds (herein sometimes referred to as “first mortgage bonds”) of the Company, issuable in series; and

WHEREAS, the Company thereafter executed and delivered certain Supplemental Indentures, First through Sixty-third, inclusive, for the various purposes of creating additional series of first mortgage bonds, conveying and confirming unto the Trustee certain additional property, correcting the description of a certain parcel of land as set forth in the Original Indenture and amending the Original Indenture in certain respects (the Original Indenture and the above referred to Supplemental Indentures together with this Sixty-fourth Supplemental Indenture being herein sometimes collectively referred to as the “Indenture”); and

WHEREAS, there have been issued and are now outstanding under the Indenture the following described first mortgage bonds:

WHEREAS, the Original Indenture in Section 158 provides that the Company, when authorized by resolution of the Board, and the Trustee, may at any time, subject to the restrictions in the Original Indenture contained, enter into such an indenture supplemental to the Original Indenture as may or shall be by them deemed necessary or desirable for the purpose of creating any new series of first mortgage bonds or of adding to the covenants and agreements of the Company in the Original Indenture contained, other covenants and agreements thereafter to be observed by the Company and for any other purpose not inconsistent with the terms of the Original Indenture and which shall not impair the security of the same; and

WHEREAS, the Company desires to execute and deliver this Sixty-fourth Supplemental Indenture, in accordance with the provisions of the Original Indenture, for the purpose of providing for the creation of a new series of first mortgage bonds to be designated “First Mortgage Bonds, Collateral Series F, Due 2014” (hereinafter called the “Bonds of Series F” or the “Bonds”), and for the purpose of adding to the covenants and agreements of the Company in the Original Indenture contained, other covenants and agreements hereafter to be observed by the Company;

WHEREAS, the Bonds are to be issued to J.P. Morgan Trust Company, National Association, as successor Trustee (the “New Mortgage Trustee”) under the Company’s Indenture of Mortgage and Deed of Trust dated as of September 1, 1993 (the “New Mortgage”), including the Sixth Supplemental Indenture thereto, dated June 9, 2005 (the “Sixth Supplemental Indenture”), and are to be owned and held by the New Mortgage Trustee as “Class ‘A’ Bonds” (as defined in the New Mortgage) in accordance with the terms of the New Mortgage; and

WHEREAS, all acts and proceedings required by law and by the Articles of Incorporation of the Company, including all action requisite on the part of its stockholders, directors and officers, necessary to make the Bonds, when executed by the Company, authenticated and delivered by the Trustee and duly issued, the valid, binding and legal obligations of the Company, and to constitute the

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Indenture a valid and binding mortgage and deed of trust for the security of the Bonds in accordance with the terms of the Indenture and the terms of the Bonds, have been done and taken; and the execution and delivery of this Sixty-fourth Supplemental Indenture have been in all respects duly authorized.

NOW, THEREFORE, THIS SIXTY-FOURTH SUPPLEMENTAL INDENTURE WITNESSETH, that, in order to further secure the payment of the principal of, premium, if any, and interest, if any, on all first mortgage bonds at any time issued and outstanding under the Indenture, according to their tenor, purport and effect, and to secure the performance and observance of all the covenants and conditions in said first mortgage bonds and in the Indenture contained (except any covenant of the Company with respect to the refund or reimbursement of taxes, assessments or other governmental charges on account of the ownership of any first mortgage bonds, or the income derived therefrom, for which the holders of such first mortgage bonds shall look only to the Company and not to the property mortgaged and pledged) and for and in consideration of the premises and of the mutual covenants herein contained and of the purchase and acceptance of the Bonds by the holders thereof, and of the sum of $1.00 duly paid to the Company by the Trustee at or before the ensealing and delivery hereof, and for other valuable considerations, the receipt whereof is hereby acknowledged, the Company has executed and delivered this Sixty-fourth Supplemental Indenture, and, by these presents does grant, bargain, sell, release, convey, assign, transfer, mortgage, pledge, set over, warrant and confirm unto the Trustee the properties of the Company described and referred to in the Original Indenture and all indentures supplemental thereto, as thereby conveyed or intended so to be, and not heretofore specifically released, together with all and singular the plants, buildings, improvements, additions, tenements, hereditaments, easements, rights, privileges, licenses and franchises and all other appurtenances whatsoever belonging or in any wise appertaining to any of the property hereby mortgaged or pledged, or intended so to be, or any part thereof, now owned or which may hereafter be owned or acquired by the Company, and the reversion and reversions, remainder and remainders, and the tolls, rents, revenues, issues, earnings, income, product and profits thereof, and of every part and parcel thereof, and all the estate, right, title, interest, property, claim and demand of every nature whatsoever of the Company, at law or in equity, or otherwise howsoever, in, of and to such property and every part and parcel thereof.

TO HAVE AND TO HOLD all and singular the lands, properties, estates, rights, franchises, privileges and appurtenances mortgaged, conveyed, pledged or assigned as aforesaid, or intended so to be, together with all the appurtenances thereunto appertaining, unto the Trustee and its successors and assigns forever, upon the trusts, for the uses and purposes and under the terms and conditions and with the rights, privileges and duties as in the Indenture set forth;

Subject, however, to the reservations, exceptions, limitations and restrictions contained in the several deeds, leases, servitudes, contracts or other instruments through which the Company acquired and/or claims title to and/or enjoys the use of the aforesaid properties; and subject also to Permitted Encumbrances (as defined in Section 24 of the Original Indenture) and, as to any property acquired by the Company since the execution and delivery of the Original Indenture, to any liens thereon existing, and to any liens for unpaid portions of the purchase money placed thereon, at the time of such acquisition, but only to the extent that such liens are permitted by Sections 72 and 83 of the Original Indenture, as amended;

BUT IN TRUST, NEVERTHELESS, for the equal and proportionate use, benefit, security and protection of those who from time to time shall hold the first mortgage bonds and coupons authenticated and delivered under the Indenture and duly issued by the Company, without any discrimination, preference or priority of any one first mortgage bond or coupon over any other by reason of priority in the time of issue, sale or negotiation thereof or otherwise, except as provided in Section 69 of the Original Indenture, so that, subject to said provisions, each and all of said first mortgage bonds and coupons shall have the same right, lien and privilege under the Indenture and shall be equally and ratably

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secured thereby (except as any sinking, amortization, improvement, renewal or other fund, or any other covenants or agreements established in accordance with the provisions of the Original Indenture, may afford additional security for the first mortgage bonds of any particular series), and shall have the same proportionate interest and share in the Trust Estate (as defined in the Original Indenture), with the same effect as if all of the first mortgage bonds and coupons had been issued, sold and negotiated simultaneously on the date of the delivery of the Original Indenture; and in trust for enforcing payment of the principal of the first mortgage bonds and of the interest and premium, if any, thereon, according to the tenor, purport and effect of the first mortgage bonds and coupons and of the Indenture, and for enforcing the terms, provisions, covenants and stipulations therein and in the first mortgage bonds set forth, and upon the trusts, uses and purposes and subject to the covenants, agreements and conditions set forth and declared in the Indenture;

AND THIS SIXTY-FOURTH SUPPLEMENTAL INDENTURE FURTHER WITNESSETH, that the Company hereby covenants and agrees to and with the Trustee and its successors and assigns forever as follows:

SECTION 1. There shall be, and is hereby created, a new series of first mortgage bonds, known as and entitled “First Mortgage Bonds, Collateral Series F, Due 2014,” and the form thereof shall be substantially as hereinafter set forth.

The Bonds of Series F shall be issued and delivered to the New Mortgage Trustee under the New Mortgage as the basis for the authentication and delivery under the New Mortgage of a series of securities (“Collateral Trust Securities”). As provided in the New Mortgage, the Bonds of Series F will be registered in the name of the New Mortgage Trustee, subject to the provisions of the New Mortgage, for the benefit of the holders of all securities from time to time outstanding under the New Mortgage, and the Company shall have no interest therein. The Bonds of Series F will not be transferable except to a successor trustee under the New Mortgage.

Any payment or deemed payment by the Company under the New Mortgage of the principal of or interest, if any, on the Collateral Trust Securities (other than by the application of the proceeds of a payment in respect of the Bonds of Series F) shall, to the extent thereof, be deemed to satisfy and discharge the obligation of the Company, if any, to make a payment of principal of or interest, if any, on such Bonds of Series F, as the case may be, which is then due.