American Century Ultra Fund - I Class (TWUIX) N-CSRS Certified semi-annual shareholder report (management investment company)

                                                              EXHIBIT 99.906CERT


     CERTIFICATION PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002
     -----------------------------------------------------------------------


     In connection with the shareholder report of American Century Mutual Funds,
Inc. (the  "Registrant") on Form N-CSR for the period ending April 30, 2007 (the
"Report"),  we,  the  undersigned,  certify,  pursuant  to  Section  906  of the
Sarbanes-Oxley Act of 2002, that:

     (1)  The Report fully  complies with the  requirements  of Section 15(d) of
          the Securities Exchange Act of 1934; and

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


Dated:  June 26, 2007

                                       /s/  Jonathan S. Thomas
                                       ------------------------------------
                                       Jonathan S. Thomas
                                       President
                                       (chief executive officer)



                                       /s/  Robert J. Leach
                                       -------------------------------------
                                       Robert J. Leach
                                       Vice President, Treasurer, and
                                       Chief Financial Officer
                                       (chief financial officer)