EXHIBIT 31.1

                      CHIEF EXECUTIVE OFFICER CERTIFICATION

     I, Scott D. Sheffield, certify that:

     I have  reviewed  this  quarterly  report on Form 10-Q of  Pioneer  Natural
Resources Company;

     Based on my knowledge, this report does not contain any untrue statement of
a  material  fact  or omit  to  state a  material  fact  necessary  to make  the
statements  made, in light of  circumstances  under which such  statements  were
made, not misleading with respect to the period covered by this report;

     Based on my  knowledge,  the  financial  statements,  and  other  financial
information included in this report, fairly present in all material respects the
financial  condition,  results of operations and cash flows of the registrant as
of, and for, the periods presented in this report;

     The  registrant's  other  certifying  officer  and  I are  responsible  for
establishing and maintaining  disclosure  controls and procedures (as defined in
Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal  control over financial
reporting  (as defined in Exchange Act Rules  13a-15(f) and  15d-15(f))  for the
registrant and have:

a)   Designed such disclosure controls and procedures, or caused such disclosure
     controls and  procedures to be designed  under our  supervision,  to ensure
     that  material  information  relating  to  the  registrant,  including  its
     consolidated  subsidiaries,  is made  known to us by  others  within  those
     entities,  particularly  during the  period in which  this  report is being
     prepared;

b)   Designed such internal  control over  financial  reporting,  or caused such
     internal  control  over  financial  reporting  to  be  designed  under  our
     supervision,  to provide reasonable  assurance regarding the reliability of
     financial  reporting  and  the  preparation  of  financial  statements  for
     external  purposes  in  accordance  with  generally   accepted   accounting
     principles;

c)   Evaluated the  effectiveness  of the registrant's  disclosure  controls and
     procedures  and  presented  in  this  report  our  conclusions   about  the
     effectiveness of the disclosure  controls and procedures,  as of the end of
     the period covered by this report based on such evaluation; and

d)   Disclosed in this report any change in the  registrant's  internal  control
     over financial  reporting that occurred during the registrant's most recent
     fiscal quarter that has  materially  affected,  or is reasonably  likely to
     materially  affect,  the  registrant's   internal  control  over  financial
     reporting; and

     The registrant's  other certifying  officer and I have disclosed,  based on
our most recent evaluation of internal control over financial reporting,  to the
registrant's  auditors  and the audit  committee  of the  registrant's  board of
directors (or persons performing the equivalent functions):

a)   All  significant  deficiencies  and  material  weaknesses  in the design or
     operation of internal control over financial reporting which are reasonably
     likely to adversely  affect the  registrant's  ability to record,  process,
     summarize and report financial information; and

b)   Any fraud,  whether or not  material,  that  involves  management  or other
     employees who have a significant role in the registrant's  internal control
     over financial reporting.

May 6, 2005


                                           /s/ Scott D. Sheffield
                                          ------------------------------------
                                          Scott D. Sheffield, Chairman and
                                          Chief Executive Officer