EXHIBIT 32.1

                         CERTIFICATION UNDER SECTION 906
                        OF THE SARBANES-OXLEY ACT OF 2002

      Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, the undersigned
certifies that this periodic report fully complies with the requirements of
Section 13(a) or 15(d) of the Securities Exchange Act of 1934 and that
information contained in this periodic report fairly presents, in all material
respects, the financial condition and results of operations of Fair Isaac
Corporation.

Date: February 8, 2005                        /s/ THOMAS G. GRUDNOWSKI
                                              ---------------------------
                                              Thomas G. Grudnowski
                                              Chief Executive Officer