SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 13D Under the Securities Exchange Act of 1934 Amendment No. 5 Pioneer Natural Resources Company ________________________________________________________ (Name of Issuer) Common Stock _______________________________________________________ (Title of Class and Securities) 723787107 _______________________________________________________ (CUSIP Number of Class of Securities) O. Mason Hawkins Chairman of the Board and C.E.O. and Andrew R. McCarroll Vice President & General Counsel Southeastern Asset Management, Inc. 6410 Poplar Avenue, Suite 900 Memphis, TN 38119 (901) 761-2474 ___________________________________________________________ (Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications) December 6, 2010 ___________________________________________ (Date of Event which Requires Filing of this Statement) If the filing person has previously filed a statement on Schedule 13G to report the acquisition which is the subject of this Schedule 13D, and is filing this Schedule because of Sections 240.13d-1(e), 240.13d-1(f), or 240.13d-1(g), check the following box: [X] CUSIP No. 723787107 13D _____________________________________________________________________________ (1) NAMES OF REPORTING PERSONS Southeastern Asset Management, Inc. I.D. No. 62-0951781 _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP: (a) (b) X _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) SOURCE OF FUNDS 00: Funds of investment advisory clients _____________________________________________________________________________ (5) CHECK BOX IF DISCOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) [ ] _____________________________________________________________________________ (6) CITIZENSHIP OR PLACE OF ORGANIZATION Tennessee _____________________________________________________________________________ :(7) SOLE VOTING POWER : (Discretionary Accounts) NUMBER OF SHARES BENEFICIALLY : 4,182,291 shares OWNED BY EACH REPORTING PERSON __________________________________________ WITH :(8) SHARED OR NO VOTING POWER : 8,255,303 shares (Shared) : 1,260,990 shares (No Vote) __________________________________________ :(9) SOLE DISPOSITIVE POWER (Discretionary Accounts) : 5,443,281 shares __________________________________________ :(10) SHARED DISPOSITIVE POWER : 8,255,303 shares (Shared) : 0 shares (None) _____________________________________________________________________________ (11) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 13,698,584 shares _____________________________________________________________________________ (12) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW 11 EXCLUDES CERTAIN SHARES _____________________________________________________________________________ (13) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 11 11.8 % _____________________________________________________________________________ (14) TYPE OF REPORTING PERSON IA _____________________________________________________________________________ CUSIP No. 723787107 13D _____________________________________________________________________________ (1) NAMES OF REPORTING PERSONS Longleaf Partners Fund I.D. No. 63-6147721 _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP: (a) (b) X _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) SOURCE OF FUNDS 00: Funds of investment company shareholders _____________________________________________________________________________ (5) CHECK BOX IF DISCLOSURE OF LEGAL PROCEDDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 29e) [ ] _____________________________________________________________________________ (6) CITIZENSHIP OR PLACE OF ORGANIZATION Massachusetts _____________________________________________________________________________ :(7) SOLE VOTING POWER NUMBER OF SHARES BENEFICIALLY : None OWNED BY EACH REPORTING PERSON __________________________________________ WITH :(8) SHARED VOTING POWER : 6,117,303 shares (Shared) __________________________________________ :(9) SOLE DISPOSITIVE POWER : None __________________________________________ :(10) SHARED DISPOSITIVE POWER : 6,117,303 shares (Shared) _____________________________________________________________________________ (11) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 6,117,303 shares _____________________________________________________________________________ (12) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW 11 EXCLUDES CERTAIN SHARES _____________________________________________________________________________ (13) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 11 5.3 % _____________________________________________________________________________ (14) TYPE OF REPORTING PERSON IV _____________________________________________________________________________ CUSIP No. 723787107 13D _____________________________________________________________________________ (1) NAMES OF REPORTING PERSONS O. Mason Hawkins I.D. No. XXX-XX-XXXX _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP: (a) (b) X _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) SOURCE OF FUNDS 00: None _____________________________________________________________________________ (5) CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) [ ] _____________________________________________________________________________ (6) CITIZENSHIP OR PLACE OF ORGANIZATION Citizen of United States _____________________________________________________________________________ :(7) SOLE VOTING POWER : (Discretionary Accounts) NUMBER OF SHARES BENEFICIALLY : None OWNED BY EACH REPORTING PERSON __________________________________________ WITH :(8) SHARED VOTING POWER : None __________________________________________ :(9) SOLE DISPOSITIVE POWER : None __________________________________________ :(10) SHARED DISPOSITIVE POWER : None _____________________________________________________________________________ (11) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON None (See Item 3) _____________________________________________________________________________ (12) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW 11 EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (13) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 11 0.0 % _____________________________________________________________________________ (14) TYPE OF REPORTING PERSON IN _____________________________________________________________________________ Item 5. Interest In Securities Of The Issuer Item 5 shall be amended and restated as follows: (a) The aggregate number and percentage of Securities to which this Schedule 13D relates is 13,698,584 shares of the common stock of the Issuer, constituting approximately 11.8% of the 116,096,190 shares outstanding. Common % of outstanding Shares Common Shares Held ___________________________________________________________________ Voting Authority Sole: 4,182,291 3.6% Shared: 8,255,303* 7.1% None: 1,260,990 1.1% Total 13,698,584 11.8% *Consists of shares owned by Longleaf Partners Fund and Longleaf Partners Small-Cap Fund, each a series of Longleaf Partners Funds Trust, an open-end management investment company registered under the Investment Company Act of 1940. Dispositive Authority Sole: 5,443,281 4.7% Shared: 8,255,303* 7.1% None: 0 0.0% Total 13,698,584 11.8% *Consists of shares owned by Longleaf Partners Fund and Longleaf Partners Small-Cap Fund, each a series of Longleaf Partners Funds Trust, an open-end management investment company registered under the Investment Company Act of 1940. (b) Southeastern generally has the sole power to dispose of or to direct the disposition of the Securities held for discretionary accounts of its investment clients, and may be granted the sole power to vote or direct the vote of such Securities; such powers may be retained by or shared with the respective clients for shared or non-discretionary accounts. Shares held by any Series of Longleaf Partners Funds Trust are reported in the "shared" category. (c) Purchase or sale transactions in the Securities during the past sixty days are disclosed on Schedule II. (d) The investment advisory clients of Southeastern have the sole right to receive and, subject to notice, to withdraw the proceeds from the sale of the Securities, and the sole power to direct the receipt of dividends from any of the Securities held for their respective accounts. Such clients may also terminate the investment advisory agreements without penalty upon appropriate notice. Southeastern does not have an economic interest in any of the Securities reported herein. (e) Not applicable. Signatures After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. Dated: December 8, 2010 Southeastern Asset Management, Inc. By /s/ Andrew R. McCarroll _______________________________________________ Andrew R. McCarroll Vice President and General Counsel Longleaf Partners Fund By: Southeastern Asset Management, Inc. /s/ Andrew R. McCarroll _______________________________________________ Andrew R. McCarroll Vice President & General Counsel O. Mason Hawkins, Individually /s/ O. Mason Hawkins _______________________________________________ Joint Filing Agreement In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, the persons or entities named below agree to the joint filing on behalf of each of them of this Amendment No. 5 to Schedule 13D with respect to the Securities of the Issuer and further agree that this joint filing agreement be included as an exhibit to this Amendment No. 5 to Schedule 13D. In evidence thereof, the undersigned hereby execute this Agreement as of December 8, 2010. Southeastern Asset Management, Inc. By /s/ Andrew R. McCarroll _______________________________________________ Andrew R. McCarroll Vice President and General Counsel Longleaf Partners Fund By: Southeastern Asset Management, Inc. /s/ Andrew R. McCarroll _______________________________________________ Andrew R. McCarroll Vice President & General Counsel O. Mason Hawkins, Individually /s/ O. Mason Hawkins _______________________________________________ SCHEDULE II Transactions in the Last Sixty Days Transaction Type Date # of Shares Price per Share* Sale 12/02/10 13,000 $84.24 Sale 12/02/10 5,300 $84.24 Sale 12/02/10 200 $84.25 Sale 12/02/10 100 $84.24 Sale 12/02/10 3,400 $84.24 Sale 12/02/10 916 $84.24 Sale 12/02/10 6,084 $84.24 Sale 12/02/10 56,000 $84.24 Sale 12/02/10 144,052 $84.24 Sale 12/02/10 9,400 $83.92 Sale 12/02/10 29,600 $84.24 Sale 12/02/10 3,200 $84.24 Sale 12/02/10 10,200 $84.24 Sale 12/02/10 10,000 $83.92 Sale 12/02/10 10,000 $84.24 Sale 12/02/10 3,000 $84.24 Sale 12/02/10 700 $84.24 Sale 12/02/10 1,300 $84.24 Sale 12/02/10 2,000 $84.24 Sale 12/03/10 2,000 $83.87 Sale 12/03/10 11,000 $83.87 Sale 12/03/10 2,000 $83.93 Sale 12/03/10 4,000 $83.93 Sale 12/03/10 5,000 $83.93 Sale 12/03/10 52,000 $83.93 Sale 12/03/10 21,000 $83.93 Sale 12/03/10 16,000 $83.93 Sale 12/03/10 2,000 $83.93 Sale 12/03/10 1,000 $83.93 Sale 12/03/10 3,000 $83.93 Sale 12/03/10 1,000 $83.93 Sale 12/03/10 5,500 $83.87 Sale 12/03/10 500 $83.87 Sale 12/03/10 5,000 $83.93 Sale 12/03/10 2,000 $83.93 Sale 12/03/10 6,000 $83.93 Sale 12/03/10 2,000 $83.93 Sale 12/03/10 1,000 $83.93 Sale 12/03/10 12,000 $83.93 Sale 12/03/10 241,016 $83.93 Sale 12/03/10 71,000 $83.87 Sale 12/03/10 62,800 $83.93 Sale 12/03/10 10,200 $83.93 Sale 12/03/10 21,000 $83.87 Sale 12/03/10 4,000 $83.93 Sale 12/03/10 4,000 $83.87 Sale 12/03/10 5,000 $83.93 Sale 12/03/10 2,000 $83.93 Sale 12/03/10 18,000 $83.87 Sale 12/03/10 7,000 $83.87 Sale 12/03/10 6,000 $83.93 Sale 12/03/10 8,000 $83.93 Sale 12/03/10 3,000 $83.93 Sale 12/03/10 2,000 $83.87 Sale 12/03/10 1,000 $83.87 Sale 12/03/10 7,000 $83.93 Sale 12/06/10 2,000 $85.01 Sale 12/06/10 3,000 $85.01 Sale 12/06/10 18,000 $85.01 Sale 12/06/10 28,000 $85.01 Sale 12/06/10 15,000 $85.01 Sale 12/06/10 2,000 $85.01 Sale 12/06/10 3,000 $85.43 Sale 12/06/10 1,000 $85.01 Sale 12/06/10 18,000 $85.01 Sale 12/06/10 27,000 $85.01 Sale 12/06/10 4,000 $85.01 Sale 12/06/10 179,069 $85.01 Sale 12/06/10 51,600 $85.01 Sale 12/06/10 1,400 $85.01 Sale 12/06/10 1,000 $85.01 Sale 12/06/10 12,900 $85.43 Sale 12/06/10 6,000 $85.01 Sale 12/06/10 2,000 $85.43 Sale 12/06/10 2,000 $85.01 Sale 12/07/10 4,000 $85.38 Sale 12/07/10 33,000 $85.68 Sale 12/07/10 6,000 $85.68 Sale 12/07/10 3,000 $85.38 Sale 12/07/10 10,000 $85.68 Sale 12/07/10 10,000 $85.38 Sale 12/07/10 164,878 $85.38 Sale 12/07/10 74,794 $85.68 Sale 12/07/10 5,000 $85.38 Sale 12/07/10 2,000 $85.68 Sale 12/07/10 10,700 $85.38 Sale 12/07/10 88,300 $85.38 Sale 12/07/10 1,600 $85.70 Sale 12/07/10 4,500 $85.68 Sale 12/08/10 12,500 $83.60 Sale 12/08/10 9,000 $83.73 Sale 12/08/10 5,000 $83.71 Sale 12/08/10 1,000 $83.73 Sale 12/08/10 5,000 $83.73 Sale 12/08/10 23,000 $83.73 Sale 12/08/10 3,000 $83.73 Sale 12/08/10 8,000 $83.60 Sale 12/08/10 1,000 $83.71 Sale 12/08/10 29,124 $83.60 Sale 12/08/10 96,000 $83.73 Sale 12/08/10 7,264 $83.71 Sale 12/08/10 7,000 $83.73 Sale 12/08/10 5,000 $83.73 Sale 12/08/10 1,000 $83.73 Sale 12/08/10 6,000 $83.73 Sale 12/08/10 1,810 $83.73 Sales by Southeastern's clients were made in the ordinary course of business through brokers and were primarily a result of the need to rebalance portfolios following an increase in the Issuer's stock price. * Net of commissions 1 SCHEDULE 13D - Pioneer Natural Resources Company ("Issuer") 2