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                                                                     Exhibit 5.1


[ROPES & GRAY LETTERHEAD]



                                                 June 28, 2006




Affiliated Managers Group, Inc.
600 Hale Street
Prides Crossing, MA 01965-1000

Ladies and Gentlemen:

         This opinion is furnished to you in connection with a registration
statement on Form S-3 (the "Registration Statement"), filed with the Securities
and Exchange Commission (the "Commission") under the Securities Act of 1933, as
amended, for the registration of (i) $291,000,000 aggregate liquidation amount
of 5.10% Convertible Trust Preferred Securities (the "Preferred Securities") of
AMG Capital Trust I, a Delaware statutory Trust (the "Trust"), (ii) $291,000,000
aggregate principal amount of Junior Subordinated Convertible Debentures due
April 15, 2036 (the "Debentures") of Affiliated Managers Group, Inc. (the
"Company"), (iii) an indeterminate number of shares of the Company's common
stock, $0.01 par value per share (the "Shares"), initially issuable upon the
conversion of the Debentures, and (iv) the guarantee by the Company of the
Preferred Securities (the "Guarantee") pursuant to the Guarantee Agreement,
dated as of April 3, 2006 between the Company and LaSalle Bank National
Association ("LaSalle Bank"), as guarantee trustee (the "Guarantee"). The
Debentures were issued under an indenture dated as of April 3, 2006 between the
Company and LaSalle Bank (the "Indenture"). All of the Debentures and Shares are
being registered on behalf of the holders of the Debentures.

         We have acted as counsel for the Company in connection with the
preparation and filing of the Registration Statement. In connection with this
opinion, we have examined the Registration Statement and the Indenture. We have
also examined such documents and made such other investigations as we have
deemed appropriate to render the opinions set forth below. As to matters of fact
material to our opinion, we have relied, without independent verification, on
representations made in the Indenture and the purchase agreement relating to the
Debentures and on certificates and other inquiries of officers of the Company
and of public officials.

         We express no opinion as to the laws of any jurisdiction other than
those of the Commonwealth of Massachusetts, the State of New York, the Delaware
General Corporate Law and the federal laws of the United States of America.

         Based upon the foregoing, we are of the opinion that:

         1. The Debentures have been duly authorized, executed and delivered by
the Company and, assuming that they have been authenticated by LaSalle Bank, as
debenture trustee, in the


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ROPES & GRAY LLP


                                       -2-                      June 28, 2006



manner prescribed by the Indenture, and delivered to and paid for by the Trust,
and the Guarantee constitute valid and binding obligations of the Company,
enforceable against the Company in accordance with their terms.

         2. The Shares have been duly authorized and, when issued upon
conversion of the Debentures or the Preferred Securities, will be validly
issued, fully paid and non-assessable.

         Our opinion in paragraph 1 is subject to bankruptcy, insolvency,
fraudulent transfer, reorganization, moratorium and other similar laws of
general application affecting the rights and remedies of creditors generally and
to general principles of equity.

         We understand that this opinion is to be used in connection with the
Registration Statement. We hereby consent to the filing of this opinion as part
of the Registration Statement and to the use of our name therein and in the
related prospectus under the caption "Validity of Securities." In giving such
consent, we do not thereby admit that we are in the category of persons whose
consent is required under Section 7 of the Securities Act.



                                                     Very truly yours,


                                                     /s/ Ropes & Gray LLP


                                                     Ropes & Gray LLP