EXHIBIT 4.1

                          SIXTH SUPPLEMENTAL INDENTURE

     THIS  SIXTH   SUPPLEMENTAL   INDENTURE  dated  as  of  May  1,  2006  (this
"Supplemental  Indenture"),  among Pioneer Natural Resources Company, a Delaware
corporation  (the  "Company"),  Pioneer Natural  Resources USA, Inc., a Delaware
corporation,  for  purposes of agreeing to make certain  guarantees  pursuant to
Section 3 hereof  (the  "Guarantor"),  and The Bank of New York  Trust  Company,
N.A.,  a national  banking  association  organized  under the laws of the United
States of America, as successor trustee (the "Trustee").  Capitalized terms used
herein and not  otherwise  defined have the meanings set forth in the  Indenture
referred to below.

                                    RECITALS

     A. The Company  and The Bank of New York,  a New York  banking  corporation
(the "Prior Trustee"),  entered into that certain Indenture, dated as of January
13, 1998 (the "Indenture"),  pursuant to which the Company may from time to time
issue  its  debentures,   notes,   bonds  or  other  evidences  of  indebtedness
(collectively, the "Debt Securities").

     B. The  Company,  the Prior  Trustee  and the  Trustee  are parties to that
certain  Agreement  of  Resignation,  Appointment  and  Acceptance,  dated as of
February 21, 2005, pursuant to which the Prior Trustee resigned as trustee under
the  Indenture  and the Trustee was  appointed  as successor  trustee  under the
Indenture.

     C. Article IX of the Indenture  provides that the Company,  when authorized
by a resolution  of the Board of Directors of the Company,  and the Trustee may,
without  the  consent  of the  holders  of the  Debt  Securities,  enter  into a
supplemental  indenture to establish the form or terms of Debt Securities of any
series as permitted by Sections 2.01 and 2.03 of the Indenture.

     D. The Company desires to issue,  and upon certain events specified in this
Supplemental  Indenture,  the  Guarantor  desires  to agree to be  obligated  to
guarantee,  $450,000,000  aggregate  principal amount of 6.875% Senior Notes due
2018 (the  "Notes") and in connection  therewith,  the Company and the Guarantor
have  duly  determined  to  make,  execute  and  deliver  to  the  Trustee  this
Supplemental  Indenture  to set forth the terms and  provisions  of the Notes as
required by the Indenture.

     NOW, THEREFORE, in consideration of the mutual agreements and covenants set
forth  herein,  the parties  hereto agree,  subject to the terms and  conditions
hereinafter set forth, as follows for the benefit of the Trustee and the Holders
of the Notes:

     Section 1. Notes. Pursuant to Section 2.03 of the Indenture,  the terms and
provisions of the Notes are as follows:

     (a)   The title of the Notes shall be "6.875% Senior Notes due 2018."

     (b)   The  Notes  shall be  initially  limited  to  $450,000,000  aggregate
principal  amount.  The Company  may,  without the consent of the Holders of the
Notes, increase such aggregate principal amount in the future, on the same terms
and conditions  and with the same CUSIP numbers as the Notes.  The Company shall





not issue any such  additional  Notes unless the  additional  Notes are fungible
with the Notes for United States federal income tax purposes.

     (c)   The Notes shall not require any  principal or  premium payments prior
to maturity on May 1, 2018.

     (d)   The rate at which the  Notes shall bear  interest shall be 6.875% per
annum;  interest  on the notes  shall  accrue  from May 1,  2006,  for the first
interest payment and from the most recent interest payment date thereafter;  the
interest  payment dates on which such  interest  shall be payable shall be May 1
and  November  1,  beginning  November  1, 2006;  and the  record  dates for the
determination of the holders of the Notes to whom such interest is payable shall
be the  immediately  preceding April 15 (for May 1 payment dates) and October 15
(for November 1 payment  dates);  the rate at which the overdue  principal shall
bear  interest  shall be 1% per annum in excess of the rate stated  initially in
this clause;  and the rate at which overdue  installments of interest shall bear
interest  shall be 1% per annum in excess of the rate stated  initially  in this
clause to the extent lawful.

     (e)   Payments of principal of and interest on the Notes represented by one
or more Global Senior Notes  initially  registered in the name of The Depository
Trust Company (the  "Depositary") or its nominee with respect to the Notes shall
be made by the Company through the Trustee in immediately available funds to the
Depositary or its nominee, as the case may be.

     (f)   The Notes  shall be  redeemable  at any  time,  at the  option of the
Company,  in whole or from time to time in part, at the price,  and otherwise in
accordance  with the  terms  and  provisions,  set  forth in  Section  2 of this
Supplemental Indenture and (to the extent they do not conflict with Section 2 of
this Supplemental  Indenture) the terms and provisions of Sections 3.03 and 3.04
of the Indenture.

     (g)   The Notes  shall be  represented by one or more  Global Senior  Notes
deposited  with the  Depositary and registered in the name of the nominee of the
Depositary.

     (h)   There  shall be no  mandatory  sinking  fund for the  payments of the
Notes.

     (i)   As long as the Depositary or its  nominee, or a  successor Depositary
or its nominee,  is the registered  owner of the Global Senior Notes relating to
the Notes, owners of the beneficial  interests in such Global Senior Notes shall
not be  entitled  to have the  Notes  registered  in their  names  and shall not
receive or be entitled to receive physical  delivery of Notes in definitive form
except (i) as provided in Section  2.15(c) of the  Indenture or (ii) if an Event
of Default with respect to the Notes has occurred and is continuing.

     (j)   The Bank of New York Trust Company, N.A. shall be the Trustee for the
Notes under the Indenture.

     (k)   Article X of the Indenture shall apply to the Notes.

     (l)   The Notes  shall not be  subordinated  pursuant  to the provisions of
Article XII of the Indenture. The Notes shall be senior unsecured obligations of
the Company  ranking pari passu with other existing and future senior  unsecured
indebtedness of the Company.

                                       2





     (m)   The  Company  shall be  subject  to all  the  covenants  set forth in
Article IV of the Indenture with respect to the Notes.

     (n)   To the extent not set forth herein, the provisions of Section 2.03 of
the Indenture are not applicable.

     Section 2. Optional  Redemption  of Notes.  The Notes will be redeemable at
any time,  at the option of the Company,  in whole or from time to time in part,
upon not less  than 30 and not more  than 60 days'  notice  as  provided  in the
Indenture,  on any date prior to  maturity  (the  "Redemption  Date") at a price
equal to 100% of the principal  amount thereof plus accrued and unpaid interest,
if any, to the Redemption Date (subject to the right of Holders of record on the
relevant record date to receive  interest due on any interest  payment date that
is on or prior to the Redemption  Date) plus a Make-Whole  Premium,  if any (the
"Redemption  Price"). In no event will a Redemption Price ever be less than 100%
of the principal amount of the Notes plus accrued and unpaid  interest,  if any,
to the Redemption Date.

     The amount of the  Make-Whole  Premium with respect to any of the Notes (or
portion thereof) to be redeemed will be equal to the excess, if any, of:

     (a)  the sum of the  present values,  calculated as of the Redemption Date,
          of:

                (i)  each interest payment that,  but for such redemption, would
     have been payable on such Note  (or portion thereof) being redeemed on each
     interest payment  date occurring after  the Redemption Date  (excluding any
     accrued interest for the period prior to the Redemption Date); and

                (ii) the principal amount that,  but for such redemption,  would
     have been payable at the final  maturity of  such Note (or portion thereof)
     being redeemed; over

     (b)  the principal amount of such Note (or portion thereof) being redeemed.

     The present values of interest and principal payments referred to in clause
(a) above will be determined in accordance with generally accepted principles of
financial  analysis.  Such present values will be calculated by discounting  the
amount of each  payment of  interest or  principal  from the date that each such
payment would have been payable, but for the redemption,  to the Redemption Date
at a discount rate equal to the Treasury  Yield (as defined below) plus 50 basis
points.

     The  Make-Whole  Premium will be  calculated by an  independent  investment
banking institution of national standing appointed by the Company; provided that
if the Company fails to make such appointment at least 45 business days prior to
the Redemption  Date, or if the  institution so appointed is unwilling or unable
to make  such  calculation,  such  calculation  will  be made by an  independent
investment banking institution of national standing appointed by the Trustee (in
any such case, an "Independent Investment Banker").

     For purposes of determining the Make-Whole Premium,  "Treasury Yield" means
a rate of interest  per annum equal to the weekly  average  yield to maturity of
United States Treasury Notes that have a constant  maturity that  corresponds to

                                       3





the  remaining  term to  maturity of the  applicable  Notes,  calculated  to the
nearest  1/12th of a year (the  "Remaining  Term").  The Treasury  Yield will be
determined as of the third  business day  immediately  preceding the  applicable
Redemption Date.

     The  weekly  average  yields  of  United  States  Treasury  Notes  will  be
determined by reference to the most recent statistical  release published by the
Federal  Reserve Bank of New York and designated  "H.15 (519) Selected  Interest
Rates" or any successor  release (the "H.15 Statistical  Release").  If the H.15
Statistical Release sets forth a weekly average yield for United States Treasury
Notes having a constant  maturity that is the same as the Remaining  Term,  then
the  Treasury  Yield will be equal to such weekly  average  yield.  In all other
cases,   the  Treasury  Yield  will  be  calculated  by   interpolation,   on  a
straight-line  basis,  between the weekly  average  yields on the United  States
Treasury  Notes that have a constant  maturity  closest to and greater  than the
Remaining  Term and the  United  States  Treasury  Notes  that  have a  constant
maturity  closest to and less than the Remaining Term (in each case as set forth
in the H.15  Statistical  Release).  Any weekly  average yields so calculated by
interpolation  will be rounded to the nearest  1/100th of 1%, with any figure of
1/200th of 1% or above being rounded upward. If weekly average yields for United
States  Treasury  Notes are not  available  in the H.15  Statistical  Release or
otherwise,  then the  Treasury  Yield will be  calculated  by  interpolation  of
comparable rates selected by the Independent Investment Banker.

     In  the  case  of any  partial  redemption,  selection  of  the  Notes  for
redemption  will be made by the Trustee on a pro rata  basis,  by lot or by such
other  method as the  Trustee in its sole  discretion  shall deem to be fair and
appropriate,  although  no such Note of $1,000 in original  principal  amount or
less shall be redeemed in part. If any Note is to be redeemed in part only,  the
notice of  redemption  relating  to such Note  shall  state the  portion  of the
principal amount thereof to be redeemed. A new Note in principal amount equal to
the unredeemed  portion thereof will be issued in the name of the Holder thereof
upon cancellation of the original Note.

     Section 3.  Obligation to Guarantee.  If at any time any of the Company's 8
1/4% Senior Notes due 2007, 6.50% Senior Notes due 2008, 5.875% Senior Notes due
2012,  5.875%  Senior  Notes due 2016 or 7.20% Senior Notes due 2028 (the "7.20%
Notes," and collectively  with all other senior notes, the "Other Senior Notes")
are  guaranteed by the  Guarantor  pursuant to the terms of the Indenture or any
applicable  supplemental  indenture  related  to such  senior  notes,  then  the
Company,  the Guarantor and the Trustee shall as soon as reasonably  practicable
thereafter  execute  and  deliver  a  supplemental  indenture  to the  Indenture
pursuant to which the  Guarantor  shall  unconditionally  guarantee the Notes on
substantially  the same terms as the Guarantor  shall have  guaranteed the 7.20%
Notes; provided, however, that if the Guarantor is not required to guarantee the
7.20% Notes or if the 7.20% Notes are no longer outstanding,  then the Guarantor
shall guarantee the Notes on  substantially  the same terms as the most recently
issued  series of Other  Senior  Notes that are  guaranteed.  The  Company,  the
Guarantor and the Trustee,  as  applicable,  also shall execute and deliver such
other  documents,  instruments or  certificates  as are reasonably  necessary or
appropriate to effect the required guarantee of the Notes.

     Section 4.  Amendments to Sections 1.01, 2.07 and 2.15 and Article IV.

     (a)  Section 1.01.  Section 1.01 is hereby amended,  solely with respect to
the Notes, by:

                                       4





                (i)  deleting clause (a)(iv)(A) of the  definition of  "Adjusted
       Consolidated  Net  Tangible  Assets"   and  substituting    therefor  the
       following  language  "the net book  value of other tangible assets of the
       Company and its  Subsidiaries,  as of a date  no earlier than the date of
       the  Company's  latest  annual or  quarterly financial  statement,  and";
       deleting   the  following  language  "Issue  Date   (including,   without
       limitation, under the Credit Agreements)" at the end of clause (e) of the
       definition of  "Permitted  Liens" and substituting therefor the following
       language  "date on which the 6.875%  Senior Notes due 2018 of the Company
       were  originally  issued";  adding  the  following  language  "and  Liens
       securing Non-Recourse Indebtedness;  provided,  however, that the related
       purchase money Indebtedness and Non-Recourse Indebtedness, as applicable,
       shall not be  secured  by any Property  or assets of the  company  or any
       Restricted  Subsidiary other than  the Property  acquired by the  Company
       with  the proceeds  of such  purchase money  Indebtedness or Non-Recourse
       Indebtedness,  as applicable"  after "Purchase Money Liens" in clause (i)
       of the definition of "Permitted Liens";

                (ii)  deleting   the  definition  of   "Credit  Agreements"  and
       substituting therefor the definition "Credit Facility" as follows:

            "Credit Facility"  means,  with  respect to the  Company, the credit
            facility made  available to the Company pursuant to  the Amended and
            Restated 5-Year Revolving  Credit Agreement  dated as  of  September
            30, 2005, among the Company and the lenders named therein,  together
            with any Refinancings thereof by a lender or a syndicate of lenders.
            It is  understood  and  agreed  that  the  Credit  Facility  may  be
            refinanced, refunded, extended, renewed  or replaced (through one or
            more  such   refinancings,  refundings,   extensions,   renewals  or
            replacements),  as a whole, or in part, from time to time  after the
            termination of the applicable Credit Facility."; and

                (iii) adding a definition of "Refinance" as follows:

            "Refinance"  means,  in respect  of any  Indebtedness, to refinance,
            extend, renew, refund, repay, prepay, redeem,  defease or retire, or
            to issue other  Indebtedness in  exchange or  replacement for,  such
            indebtedness.  "Refinanced" and "Refinancing" shall have correlative
            meanings.

                (iv) adding  a  definition  of  "Non-Recourse  Indebtedness"  as
       follows:

            "Non-Recourse  Indebtedness"  means  Indebtedness or that portion of
            Indebtedness  of  the  Company  incurred   in  connection  with  the
            acquisition by the Company of any Property and as to which:

                (1) the holders of such Indebtedness  agree in writing that they
       will  look  solely  to  the   Property  so  acquired  and  securing  such
       Indebtedness for payment on or in respect of such Indebtedness and

                                       5






                (2) no default  with respect  to such  Indebtedness would permit
       (after notice or passage of time or both),  according to the terms of any
       other Indebtedness of the Company or a Restricted Subsidiary,  any holder
       of  such  other  Indebtedness  to  declare  a  default  under such  other
       Indebtedness  or  cause  the  payment of  such other  Indebtedness  to be
       accelerated or payable prior to its stated maturity.

     (b)   Sections 2.07. Section 2.07 is hereby amended, solely with respect to
the Notes,  by adding  "the  Underwriters,"  before  "the  Company"  in the last
sentence of Section 2.07.

     (c)   Sections 2.15. Section 2.15 is hereby amended, solely with respect to
the Notes,  by adding  "the  Underwriters,"  before  "the  Company" in the third
sentence of subclause (v) of Section 2.15(c).

     Section 5. Ratification.  This Supplemental Indenture is executed and shall
be construed as an indenture  supplemental  to the Indenture and, as provided in
the Indenture, this Supplemental Indenture forms a part of the Indenture. Except
to the extent amended by or supplemented  by this  Supplemental  Indenture,  the
Company,  the Guarantor and the Trustee hereby ratify,  confirm and reaffirm the
Indenture in all respects.

     Section 6. Counterparts. This Supplemental Indenture may be executed in any
number of counterparts,  each of which so executed shall be an original, but all
such counterparts shall together constitute but one and the same instrument.

     Section 7.  Governing  Law.  The laws of the State of New York shall govern
the construction and  interpretation  of this  Supplemental  Indenture,  without
regard to principles of conflicts of laws.

     Section 8. Trustee Not Responsible  for Recitals or Issuance of Notes.  The
recitals contained herein and in the Notes, except the Trustee's certificates of
authentication, shall be taken as the statements of the Company, and the Trustee
assumes  no  responsibility  for  their   correctness.   The  Trustee  makes  no
representations as to the validity or sufficiency of this Supplemental Indenture
or of the Notes. The Trustee shall not be accountable for the use or application
by the Company of Notes or the proceeds thereof.



                            [Signature Page Follows]


                                       6








     IN WITNESS  WHEREOF,  the parties  hereto  have  caused  this  Supplemental
Indenture to be signed on their behalf by their duly authorized  representatives
as of the date first above written:



                                            Pioneer Natural Resources Company



                                      By:  /s/ Richard P. Dealy
                                           ------------------------------------
                                      Name:  Richard P. Dealy
                                      Title: Executive Vice President and
                                             Chief Financial Officer




                                            Pioneer Natural Resources USA, Inc.


                                      By:  /s/ Richard P. Dealy
                                           ------------------------------------
                                      Name:  Richard P. Dealy
                                      Title: Executive Vice President and
                                             Chief Financial Officer




                                      The Bank of New York Trust Company, N.A.,
                                      as Trustee


                                      By:  /s/ John C. Stohlmann
                                           ------------------------------------
                                      Name:  John C. Stohlmann
                                      Title: Vice President