Exhibit 4.5

REGISTRATION RIGHTS AGREEMENT

THIS REGISTRATION RIGHTS AGREEMENT (this “Agreement”) is made and entered into as of February 12, 2004, by and between AFFILIATED MANAGERS GROUP, INC., a Delaware corporation (the “Company”) and MERRILL LYNCH & CO., MERRILL LYNCH, PIERCE, FENNER & SMITH INCORPORATED (“Merrill Lynch”).

This Agreement is made pursuant to the Purchase Agreement, dated February 6, 2004 (the “Purchase Agreement”), between the Company, as issuer of [220,000] PRIDES ^SM (the “PRIDES”) each with a stated amount of $1,000, which will initially consist of units (referred to as “Income PRIDES ^SM ”), comprised of (a) a stock purchase contract (the “Purchase Contract”) under which (i) the holder will purchase from the Company on February 17, 2008 a number of shares of common stock, par value $0.01 per share, of the Company (the “Common Stock”) equal to the Settlement Rate as set forth in the Purchase Contract Agreement (as defined below) and (ii) the Company will pay certain Contract Adjustment Payments to the holders as provided in the Purchase Contract Agreement, and (b) $1,000 principal amount of the Company’s 4.125% Senior Notes due February 17, 2010, and Merrill Lynch, which provides for, among other things, the sale by the Company to Merrill Lynch of the aggregate principal amount of PRIDES specified therein.  In order to induce Merrill Lynch to enter into the Purchase Agreement, the Company has agreed to provide the registration rights set forth in this Agreement. The execution of this Agreement is a condition to the closing under the Purchase Agreement.

The Company agrees with Merrill Lynch, (i) for its benefit as a Holder of the PRIDES and (ii) for the benefit of the beneficial owners (including Merrill Lynch) from time to time of the PRIDES, and the beneficial owners from time to time of the Underlying Common Stock (as defined herein) issued upon settlement of the Purchase Contracts (each of the foregoing a “Holder” and together the “Holders”), as follows:

SECTION 1.            Definitions .  Capitalized terms used herein without definition shall have their respective meanings set forth in the Purchase Agreement.  As used in this Agreement, the following terms shall have the following meanings:

“Additional Shares:”  See Section 2(f) hereof.

“Business Day:”  Each Monday, Tuesday, Wednesday, Thursday and Friday that is not a day on which banking institutions in The City of New York are authorized or obligated by law or executive order to close.

“Common Stock:”  See the second paragraph hereof.

“Company:”  See the first paragraph hereof.

“Deferral Notice:”  See Section 3(h) hereof.

“Deferral Period:”  See Section 3(h) hereof.

“Effectiveness Deadline Date:”  See Section 2(a) hereof.

“Effectiveness Period:”  The period beginning on February 17, 2008 and ending on the earlier of (i) the sale pursuant to the Initial Shelf Registration Statement of all Registrable Securities thereunder and (ii) the expiration of the holding period applicable to such Registrable Securities held by persons not Affiliates of the Company under Rule 144(k) under the Securities Act.

“Event:”  See Section 2(f) hereof.

“Event Date:”  See Section 2(f) hereof.

“Exchange Act:”  The Securities Exchange Act of 1934, as amended, and the rules and regulations of the SEC promulgated thereunder.

“Holder:”  See the third paragraph hereof.

“Initial Shelf Registration Statement:”  See Section 2(a) hereof.

“Issue Date:” means February 12, 2004.

“Material Event:”  See Section 3(h) hereof.

“Notice and Questionnaire:”  A written notice delivered to the Company containing substantially the information called for by the Selling Securityholder Notice and Questionnaire attached as Annex A to the Offering Memorandum of the Company, dated February 12, 2004, relating to the PRIDES.

“Notice Holder:”  On any date, any Holder that has delivered a Notice and Questionnaire to the Company on or prior to such date.

“PRIDES:”  See the second paragraph hereof.

“Prospectus:”  The prospectus included in any Registration Statement (including, without limitation, a prospectus that discloses information previously omitted from a prospectus filed as part of an effective registration statement in reliance upon Rule 415 promulgated under the Securities Act), as amended or supplemented by any amendment or prospectus supplement, including post-effective amendments, and all materials incorporated by reference in such Prospectus.

“Purchase Agreement:”  See the second paragraph hereof.

“Purchase Contract Agreement:” means the purchase contract agreement to be dated as of February 12, 2004, between the Company and The Bank of New York, as purchase contract agent.

“Record Holder:”  With respect to an Event Date, the registered holder of such Registrable Security.

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“Registrable Securities:”  The Underlying Common Stock, until the earliest of (i) its effective registration under the Securities Act and resale in accordance with the Registration Statement covering it, (ii) its sale to the public pursuant to Rule 144, or (iii) the expiration of the Effectiveness Period.

“Registration Statement:”  Any registration statement of the Company that covers any of the Registrable Securities pursuant to the provisions of this Agreement, including the Prospectus, amendments and supplements to such registration statement, including post-effective amendments, all exhibits, and all materials incorporated by reference in such registration statement.

“Rule 144:”  Rule 144 under the Securities Act, as such Rule may be amended from time to time, or any similar rule or regulation hereafter adopted by the SEC.

“SEC:”  The Securities and Exchange Commission.

“Securities Act:”  The Securities Act of 1933, as amended, and the rules and regulations promulgated by the SEC thereunder.

“Settlement Rate:”  See the second paragraph hereof.

“Shelf Registration Statement:”  See Section 2(a) hereof.

“Subsequent Shelf Registration Statement:”  See Section 2(c) hereof.

“TIA:”  The Trust Indenture Act of 1939, as amended.

“Underlying Common Stock:”  The Common Stock which is delivered upon settlement of the Purchase Contracts.

SECTION 2.            Shelf Registration .

(a)            The Company shall prepare or cause to be prepared and shall use reasonable efforts to file or cause to be filed with the SEC a Registration Statement for an offering to be made on a delayed or continuous basis pursuant to Rule 415 of the Securities Act (a “Shelf Registration Statement”) registering the resale from time to time by Holders thereof of all of the Registrable Securities (the “Initial Shelf Registration Statement”). The Initial Shelf Registration Statement shall be on Form S-3 or another appropriate form permitting registration of such Registrable Securities for resale by such Holders in accordance with the methods of distribution elected by the Holders and set forth in the Initial Shelf Registration Statement; provided, that in no event will such method(s) of distribution take the form of an underwritten offering of the Registrable Securities without the prior agreement of the Company.  The Company shall use reasonable efforts to cause the Initial Shelf Registration Statement to be declared effective under the Securities Act by a date (the “Effectiveness Deadline Date”) that is no later than the Business Day prior to February 17, 2008 and to keep the Initial Shelf Registration Statement (or any Subsequent Shelf Registration Statement) continuously effective under the Securities Act until the expiration of the Effectiveness Period, subject to the rights of the Company under Section 3(h) to create a Deferral Period. At the time the Initial Shelf

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Registration Statement is declared effective, each Holder that became a Notice Holder on or prior to the date ten (10) Business Days prior to such time of effectiveness shall be named as a selling securityholder in the Initial Shelf Registration Statement and the related Prospectus in such a manner as to permit such Holder to deliver such Prospectus to purchasers of Registrable Securities in accordance with applicable law.

(b)            Upon the occurrence of an Early Settlement, a Merger Early Settlement or a Significant Corporate Action Early Settlement (as each term is defined in the Purchase Contract Agreement), the Company shall prepare or cause to be prepared and shall use reasonable efforts to file or cause to be filed with the SEC the Initial Shelf Registration Statement within ten (10) Business Days after the occurrence of such early settlement and to cause the Initial Shelf Registration Statement to be declared effective under the Securities Act as soon as practicable thereafter.  The Company shall keep the Initial Shelf Registration Statement (or any Subsequent Shelf Registration Statement) continuously effective under the Securities Act until the expiration of the Effectiveness Period, subject to the rights of the Company under Section 3(h) to create a Deferral Period.

(c)            If the Initial Shelf Registration Statement or any Subsequent Shelf Registration Statement ceases to be effective for any reason at any time during the Effectiveness Period, the Company shall use reasonable efforts to obtain the prompt withdrawal of any order suspending the effectiveness thereof, and in any event shall within thirty (30) days of such cessation of effectiveness amend the Shelf Registration Statement in a manner reasonably expected by the Company to obtain the withdrawal of the order suspending the effectiveness thereof, or file an additional Shelf Registration Statement covering all of the Securities that as of the date of such filing are Registrable Securities (a “Subsequent Shelf Registration Statement”).  If a Subsequent Shelf Registration Statement is filed, the Company shall use reasonable efforts to cause the Subsequent Shelf Registration Statement to become effective as promptly as is practicable after such filing and to keep such Registration Statement (or subsequent Shelf Registration Statement) continuously effective until the end of the Effectiveness Period.

(d)            The Company shall supplement and amend the Shelf Registration Statement if required by the rules, regulations or instructions applicable to the registration form used by the Company for such Shelf Registration Statement if required by the Securities Act or, to the extent to which the Company does not reasonably object, as reasonably requested by Merrill Lynch.

(e)            Each Holder of Underlying Common Stock agrees that if such Holder wishes to sell Underlying Common Stock pursuant to a Shelf Registration Statement and related Prospectus, it will do so only in accordance with this Section 2(e) and Section 3(h).  Each Holder of Underlying Common Stock wishing to sell Underlying Common Stock pursuant to a Shelf Registration Statement and related Prospectus agrees to deliver a Notice and Questionnaire to the Company at least eight (8) but not more than twenty (20) Business Days prior to any intended resale of Underlying Common Stock under the Shelf Registration Statement. From and after the date the Initial Shelf Registration Statement is declared effective, the Company shall, as promptly as reasonably practicable after the date a Notice and Questionnaire is delivered, and in any event within five (5) Business Days after such date, (i) if required by applicable law, file with the SEC a post-effective amendment to the Shelf Registration Statement or prepare and, if

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required by applicable law, file a supplement to the related Prospectus or a supplement or amendment to any document incorporated therein by reference or file any other document required by the SEC so that the Holder delivering such Notice and Questionnaire is named as a selling securityholder in the Shelf Registration Statement and the related Prospectus in such a manner as to permit such Holder to deliver such Prospectus to purchasers of the Underlying Common Stock in accordance with applicable law and, if the Company shall file a post-effective amendment to the Shelf Registration Statement, use reasonable efforts to cause such post-effective amendment to be declared effective under the Securities Act as promptly as is practicable; (ii) provide such Holder copies of any documents filed pursuant to Section 2(e)(i); and (iii) notify such Holder as promptly as practicable after the effectiveness under the Securities Act of any post-effective amendment filed pursuant to Section 2(e)(i); provided, that if such Notice and Questionnaire is delivered during a Deferral Period, the Company shall so inform the Holder delivering such Notice and Questionnaire and shall take the actions set forth in clauses (i), (ii) and (iii) above upon expiration of the Deferral Period in accordance with Section 3(h).  Notwithstanding anything contained herein to the contrary, the Company shall be under no obligation to name any Holder that is not a Notice Holder as a selling securityholder in any Registration Statement or related Prospectus; provided, however, that any Holder that becomes a Notice Holder pursuant to the provisions of Section 2(e) of this Agreement (whether or not such Holder was a Notice Holder at the time the Registration Statement was initially declared effective) shall be named as a selling securityholder in the Registration Statement or related Prospectus subject to and in accordance with the requirements of this Section 2(e).

(f)             The parties hereto agree that the Holders of Underlying Common Stock will suffer damages, and that it would not be feasible to ascertain the extent of such damages with precision, if (i) the Initial Shelf Registration Statement has not been declared effective under the Securities Act on or prior to the Effectiveness Deadline Date, or (ii) the aggregate duration of Deferral Periods in any period exceeds the number of days permitted in respect of such period pursuant to Section 3(h) hereof (each of the events of a type described in any of the foregoing clauses (i) and (ii) are individually referred to herein as an “Event,” and the Effectiveness Deadline Date in the case of clause (i), and the date on which the aggregate duration of Deferral Periods in any period exceeds the number of days permitted by Section 3(h) hereof in the case of clause (ii), being referred to herein as an “Event Date”).

Accordingly, on any Event Date with respect to Record Holders of Underlying Common Stock, the Company agrees to deliver to such Holders, as damages and not as a penalty, a number of additional shares (the “Additional Shares”), deliverable on February 17, 2008, upon the occurrence of an Event described in 2(f)(i) or within ten (10) Business Days of the occurrence of an Event described in 2(f)(ii) to Record Holders of Underlying Common Stock, equal to 2.0% of the number of shares of Underlying Common Stock held by such Record Holder; provided however, if a Holder of Underlying Common Stock has not delivered a Notice and Questionnaire to the Company pursuant to Section 2(e) indicating that they wish to sell their Common Stock pursuant to the Initial Shelf Registration Statement, such Holder will not be entitled to receive the Additional Shares.  Notwithstanding the foregoing, the parties agree that the sole remedy for a violation of the terms of this Agreement shall be the delivery of the Additional Shares.

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Notwithstanding the foregoing, no Additional Shares shall be delivered with respect to any Underlying Common Stock after (x) the date such security is no longer a Registrable Security and (y) the expiration of the Effectiveness Period.

All of the Company’s obligations set forth in this Section 2(f) that are outstanding with respect to any Underlying Common Stock at the time such security ceases to be a Registrable Security shall survive until such time as all such obligations with respect to such security have been satisfied in full (notwithstanding termination of this Agreement pursuant to Section 7(j)).

The parties hereto agree that the Additional Shares provided for in this Section 2(f) constitute a reasonable estimate of the damages that may be incurred by Record Holders of Underlying Common Stock that are Registrable Securities by reason of the failure of the Shelf Registration Statement to be declared effective or available for effecting resales of Registrable Securities in accordance with the provisions hereof.

SECTION 3.            Registration Procedures .  In connection with the registration obligations of the Company under Section 2 hereof, the Company shall:

(a)            Before filing any Registration Statement or Prospectus or any amendments or supplements thereto with the SEC, furnish to Merrill Lynch copies of all such documents proposed to be filed and give reasonable consideration to such comments as Merrill Lynch reasonably shall propose within three (3) Business Days of the delivery of such copies to Merrill Lynch.

(b)            Subject to Section 3(h), prepare and file with the SEC such amendments and post-effective amendments to each Registration Statement as may be necessary to keep such Registration Statement continuously effective for the applicable period specified in Section 2(a); cause the related Prospectus to be supplemented by any required Prospectus Supplement, and as so supplemented to be filed pursuant to Rule 424 (or any similar provisions then in force) under the Securities Act; and use reasonable efforts to comply with the provisions of the Securities Act applicable to it with respect to the disposition of all securities covered by such Registration Statement during the Effectiveness Period in accordance with the intended methods of disposition by the sellers thereof set forth in such Registration Statement as so amended or such Prospectus as so supplemented.

(c)            As promptly as practicable give notice to the Notice Holders and Merrill Lynch (i) when any Prospectus, Prospectus Supplement, Registration Statement or post-effective amendment to a Registration Statement (other than any such Prospectus Supplement, Registration Statement or post-effective amendment to a Registration Statement which is filed solely to name additional selling security holders or to reflect any other matters that are not of a material nature) has been filed with the SEC and, with respect to a Registration Statement or any post-effective amendment, when the same has been declared effective, (ii) of any request, following the effectiveness of the Initial Shelf Registration Statement under the Securities Act, by the SEC or any other federal or state governmental authority for amendments or supplements to any Registration Statement or related Prospectus or for additional information, (iii) of the issuance by the SEC or any other federal or state governmental authority of any stop order

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suspending the effectiveness of any Registration Statement or the initiation or threatening of any proceedings for that purpose, and (iv) of the receipt by the Company of any notification with respect to the suspension of the qualification or exemption from qualification of any of the Registrable Securities for sale in any jurisdiction or the initiation or threatening of any proceeding for such purpose, which notice may, at the discretion of the Company (or as required pursuant to Section 3(h)), state that it constitutes a Deferral Notice, in which event the provisions of Section 3(h) shall apply.

(d)            Use all reasonable efforts to obtain the withdrawal of any order suspending the effectiveness of a Registration Statement or the lifting of any suspension of the qualification (or exemption from qualification) of any of the Registrable Securities for sale in any jurisdiction in which they have been qualified for sale, in either case at the earliest possible moment.

(e)            As promptly as reasonably practicable furnish to each Notice Holder and Merrill Lynch, upon their request and without charge, at least one (1) conformed copy of the Registration Statement and any amendment thereto, including financial statements, but excluding schedules, all documents incorporated or deemed to be incorporated therein by reference and all exhibits (unless requested in writing to the Company by such Notice Holder or Merrill Lynch, as the case may be).

(f)             During the Effectiveness Period, deliver to each Notice Holder in connection with any sale of Underlying Common Stock pursuant to a Registration Statement, without charge, as many copies of the Prospectus or Prospectuses relating to such Registrable Securities (including each preliminary prospectus) and any amendment or supplement thereto as such Notice Holder may reasonably request; and the Company hereby consents (except during such periods that a Deferral Notice is outstanding and has not been revoked) to the use of such Prospectus or each amendment or supplement thereto by each Notice Holder in connection with any offering and sale of the Underlying Common Stock covered by such Prospectus or any amendment or supplement thereto in the manner set forth therein.

(g)            Subject to Section 3(h), prior to any public offering of the Registrable Securities pursuant to the Shelf Registration Statement, use all reasonable efforts to register or qualify or cooperate with the Notice Holders in connection with the registration or qualification (or exemption from such registration or qualification) of such Registrable Securities for offer and sale under the securities or Blue Sky laws of such jurisdictions within the United States as any Notice Holder reasonably requests in writing (which request may be included in the Notice and Questionnaire); prior to any public off