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                                                                    EXHIBIT 32.1

                            CERTIFICATION PURSUANT TO
                             18 U.S.C. SECTION 1350,
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

      In connection with the Annual Report on Form 10-K of Affiliated Managers
Group, Inc. (the "Company") for the period ended December 31, 2003 as filed with
the Securities and Exchange Commission on the date hereof (the "Report"), the
undersigned, William J. Nutt, Chairman and Chief Executive Officer of the
Company, hereby certifies, pursuant to 18 U.S.C. Section 1350, that:

            (1) the Report fully complies with the requirements of Section 13(a)
      or 15(d), as applicable, of the Securities Exchange Act of 1934, as
      amended; and

            (2) the information contained in the Report fairly presents, in all
      material respects, the financial condition and results of operations of
      the Company.

Date: March 15, 2004


                                    By: /s/             William J. Nutt
                                        ----------------------------------------
                                                        WILLIAM J. NUTT
                                           CHAIRMAN AND CHIEF EXECUTIVE OFFICER