EXHIBIT 32.1



                                CERTIFICATION OF
                             CHIEF EXECUTIVE OFFICER
                      OF PIONEER NATURAL RESOURCES COMPANY
                         PURSUANT TO 18 U.S.C. ss. 1350


     I, Scott D. Sheffield,  Chairman,  President and Chief Executive Officer of
Pioneer  Natural  Resources  Company (the  "Company"),  hereby  certify that the
accompanying  report on Form 10-Q for the quarterly  period ended March 31, 2005
and  filed  with the  Securities  and  Exchange  Commission  on the date  hereof
pursuant to Section 13(a) of the Securities  Exchange Act of 1934 (the "Report")
by the Company fully complies with the requirements of that section.

     I further  certify  that the  information  contained  in the Report  fairly
presents,  in all  material  aspects,  the  financial  condition  and results of
operations of the Company.




                                            /s/ Scott D. Sheffield
                                          ------------------------------------
                                    Name:   Scott D. Sheffield, Chairman and
                                              Chief Executive Officer
                                    Date:   May 6, 2005