<PAGE>

                                                                   Exhibit 10.20
================================================================================


                          NEWS AMERICA INCORPORATED,

                                              Company,

                         THE NEWS CORPORATION LIMITED,
                           FOX BROADCASTING COMPANY,
                        FOX ENTERTAINMENT GROUP, INC.,
                        FOX TELEVISION HOLDINGS, INC.,
                        HARPERCOLLINS PUBLISHERS INC.,
                              HARPERCOLLINS (UK),
                     THE HERALD AND WEEKLY TIMES LIMITED,
                         NATIONWIDE NEWS PTY. LIMITED,
                       NEWS AMERICA MARKETING FSI, INC.,
                        NEWS GROUP NEWSPAPERS LIMITED,
                            NEWS INTERNATIONAL plc,
                                 NEWS LIMITED,
                      NEWS PUBLISHING AUSTRALIA LIMITED,
                             NEWS SECURITIES B.V.,
                           NEWS T INVESTMENTS, INC.,
                         NEWS TRIANGLE FINANCE, INC.,
                             NEWSCORP INVESTMENTS,
                          NEWSCORP OVERSEAS LIMITED,
                    TWENTIETH CENTURY FOX FILM CORPORATION,
                TWENTIETH CENTURY FOX HOME ENTERTAINMENT, INC.,

                                              Guarantors
                                      and

                             THE BANK OF NEW YORK,

                                              Trustee
                              __________________

                         SIXTH SUPPLEMENTAL INDENTURE
                           Dated as of March 1, 1999

                        Supplementing and Amending the
                        Amended and Restated Indenture
                          Dated as of March 24, 1993
                              __________________

                               Senior Securities

================================================================================
<PAGE>

     SIXTH SUPPLEMENTAL INDENTURE, dated as of March 1, 1999, among News America
Incorporated, a Delaware corporation ("NAI" or the "Company") with its principal
office located at 1211 Avenue of the Americas, New York, NY 10036, The News
Corporation Limited, a South Australia, Australia corporation (A.C.N. 007 910
330) ("News Corporation"), Fox Broadcasting Company, Fox Entertainment Group,
Inc., Fox Television Holdings, Inc., HarperCollins Publishers Inc.,
HarperCollins (UK), The Herald and Weekly Times Limited, Nationwide News Pty.
Limited, News America Marketing FSI, Inc., News Group Newspapers Limited, News
International plc, News Limited, News Publishing Australia Limited, News
Securities B.V., News T Investments, Inc., News Triangle Finance, Inc., Newscorp
Investments, Newscorp Overseas Limited, Twentieth Century Fox Film Corporation,
Twentieth Century Fox Home Entertainment, Inc., and The Bank of New York, a New
York banking corporation (the "Trustee"), supplementing and amending the Amended
and Restated Indenture, dated as of March 24, 1993 (the "Original Indenture"),
among the Company, the guarantors named therein (collectively, the "Guarantors")
and the Trustee, which provided for the issuance from time to time of the
Company's senior debt securities, to be issued in one or more series as provided
therein (the "Securities"). (The Original Indenture as supplemented by the First
Supplemental Indenture, dated as of May 20, 1993, the Second Supplemental
Indenture, dated as of May 28, 1993, the Third Supplemental Indenture, dated as
of July 21, 1993, the Fourth Supplemental Indenture, dated as of October 20,
1995, the Fifth Supplemental Indenture, dated as of January 8, 1998, and this
Sixth Supplemental Indenture and as may hereafter be supplemented is referred to
herein as the or this "Indenture"). Capitalized terms used herein and not
otherwise defined herein have the meanings ascribed thereto in the Indenture.


                                   RECITALS:

     WHEREAS, effective March 1, 1999, all of the stock of News America
Publications Inc., a Guarantor under the Indenture, was sold by News
Corporation, and upon such sale, News America Publications Inc. ceased to be a
Guarantor under Indenture; and

     WHEREAS, effective June 30, 1998, News America FSI, Inc., a Guarantor under
the Indenture, changed its name to News America Marketing FSI, Inc.; effective
August 11, 1998, Twentieth Holdings Corporation, a Guarantor under the
Indenture, changed its name to Fox Entertainment Group, Inc.; effective October
24, 1998, Fox Television Stations, Inc., a Guarantor under the Indenture,
changed its name to Fox Television Holdings, Inc.; and effective January 22,
1999, Newscorp Investments Limited, a Guarantor under the Indenture, changed its
name to Newscorp Investments; and

     WHEREAS, the provisions of this Sixth Supplemental Indenture shall not
adversely affect the interests of the Holders of Securities of any series in any
material respect; and

     WHEREAS, the Original Indenture is subject to the provisions of the United
States Trust Indenture Act of 1939, as amended (the "TIA"), that are required to
be part of the Original Indenture and the Indenture shall, to the extent
applicable, be governed by such provisions; and

                                      -2-
<PAGE>

     WHEREAS, the Company has duly authorized the execution and delivery of this
Sixth  Supplemental Indenture and has done all things necessary to make this
Sixth Supplemental Indenture a valid agreement in accordance with its terms.

     NOW, THEREFORE, THIS INDENTURE WITNESSETH:

     For and in consideration of the premises and the purchase of the Securities
by the Holders thereof, it is mutually covenanted and agreed, for the equal and
proportionate benefit of all Holders of the Securities or of any series thereof,
as follows:


                                  ARTICLE ONE

                              ORIGINAL INDENTURE

     SECTION 101.   Effect of Original Indenture.
                    ----------------------------

     Except as specifically provided in this Sixth Supplemental Indenture, the
Original Indenture shall remain in full force and effect.


                                  ARTICLE TWO

                          AMENDMENTS TO THE INDENTURE

     SECTION 201.

     In accordance with Section 801 of the Original Indenture, News America
Publications Inc., incorporated in the State of Delaware, is hereby eliminated
as a Guarantor under the Indenture.

     SECTION 202.   Change of Name of Certain Guarantors.
                    ------------------------------------

     In accordance with Section 801 of the Original Indenture, the name of News
America FSI, Inc. is hereby changed to News America Marketing FSI, Inc., the
name of Twentieth Holdings Corporation is hereby changed to Fox Entertainment
Group, Inc., the name of Fox Television Stations, Inc. is hereby changed to Fox
Television Holdings, Inc. and the name of Newscorp Investments Limited is hereby
changed to Newscorp Investments.


     SECTION 203.   References in the Indenture.
                    ---------------------------

     By reason of the elimination of News America Publications Inc. as a
Guarantor pursuant to Section 201 hereof, the name change of four Guarantors
pursuant to Section 202 hereof and the continuation, as Guarantors, of the
Guarantors under the Indenture, all references in the Indenture to the
"Guarantors" are hereby deemed to refer to the following entities and all such
references to each or any "Guarantor" are hereby deemed to refer to each of such
entities:

                                      -3-
<PAGE>

     Name                               Jurisdiction of Incorporation
     ----                               -----------------------------

The News Corporation Limited                      South Australia, Australia
Fox Broadcasting Company                          Delaware
Fox Entertainment Group, Inc.                     Delaware
Fox Television Holdings, Inc.                     Delaware
HarperCollins Publishers Inc.                     Delaware
HarperCollins (UK)                                England
The Herald and Weekly Times Limited               Victoria, Australia
Nationwide News Pty. Limited                      Australian Capital
                                                   Territory, Australia
News America Marketing FSI, Inc.                  Delaware
News Group Newspapers Limited                     England
News International plc                            England
News Limited                                      South Australia, Australia
News Publishing Australia Limited                 Delaware
News Securities B.V.                              Netherlands
News T Investments, Inc.                          Delaware
News Triangle Finance, Inc.                       Delaware
Newscorp Investments                              England
Newscorp Overseas Limited                         Cayman Islands,
                                                   British West Indies
Twentieth Century Fox Film Corporation            Delaware
Twentieth Century Fox Home Entertainment, Inc.    Michigan


     SECTION 204.   Change of Definition of "Fox Group."
                    ----------------------------------

     In accordance with Section 801 of the Original Indenture, the definition of
"Fox Group" set forth in Section 101 of the Original Indenture is hereby amended
by deleting the existing definition thereof and substituting therefor the
following:

          "Fox Group" means Fox Entertainment Group, Inc., a Delaware
          corporation and its respective consolidated Restricted Subsidiaries.

     SECTION 205.   Change of Definition of "Subsidiary."
                    -----------------------------------

     In accordance with Section 801 of the Original Indenture, the definition of
"Subsidiary" set forth in Section 101 of the Original Indenture is hereby
amended by deleting the existing definition thereof and substituting therefor
the following:

     "Subsidiary" is defined as, with respect to any Person, (i) a corporation a
     majority of whose capital stock with voting power, under ordinary
     circumstances, to elect directors is at the time, directly or indirectly,
     owned by such Person, by one or more Subsidiaries of such Person or by such
     Person and one or more Subsidiaries thereof or (ii) any other Person (other
     than a corporation) in which such Person,

                                      -4-
<PAGE>

     one or more Subsidiaries thereof or such Person and one or more
     Subsidiaries thereof, directly or indirectly, at the date of determination
     thereof has at least a majority ownership interest and the power to direct
     the policies, management and affairs thereof and shall, with respect to
     News Corporation and NAI, include Fox Television Holdings, Inc. and its
     Subsidiaries and their successors. For purposes of this definition, any
     director's qualifying shares or investments by foreign nationals mandated
     by applicable law shall be disregarded in determining the ownership of a
     Subsidiary.


                                 ARTICLE THREE

                                 MISCELLANEOUS

     SECTION 301.   Effect of Headings.
                    ------------------

     The Article and Section headings herein are for convenience of reference
only and shall not effect the construction hereof.

     SECTION 302.   Governing Law.
                    -------------

     Subject to the following sentence, this Sixth Supplemental Indenture shall
be governed by, and construed in accordance with, the laws of the State of New
York, without regard to principles of conflicts of laws.  This Sixth
Supplemental Indenture is subject to the provisions of the TIA that are required
to be part of the Original Indenture and shall, to the extent applicable, be
governed by such provisions.


     SECTION 303.   Counterparts.
                    ------------

     This Sixth Supplemental Indenture may be executed in any number of
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

     IN WITNESS WHEREOF, the parties hereto have caused this Sixth Supplemental
Indenture to be duly executed as of the day and year first above written.

                                 News America Incorporated



                                 By: /s/ Arthur M. Siskind
                                    --------------------------------
                                    Name:  Arthur M. Siskind
                                    Title: Senior Executive Vice President

                                      -5-
<PAGE>

                                 Executed as a Deed in New York,
                                 New York


                                 The News Corporation Limited,
                                        as Guarantor


                                 Fox Broadcasting Company,
                                        as Guarantor


                                 Fox Entertainment Group, Inc.
                                        as Guarantor


                                 Fox Television Holdings, Inc.,
                                        as Guarantor


                                 HarperCollins Publishers Inc.,
                                        as Guarantor


                                 HarperCollins (UK),
                                        as Guarantor


                                 The Herald and Weekly Times Limited,
                                        as Guarantor


                                 Nationwide News Pty. Limited,
                                        as Guarantor


                                 News America Marketing FSI, Inc.,
                                        as Guarantor


                                 News Group Newspapers Limited,
                                        as Guarantor


                                 News International plc,
                                        as Guarantor

                                      -6-
<PAGE>

                                 News Limited,
                                        as Guarantor

                                 News Publishing Australia Limited,
                                        as Guarantor

                                 News Securities B.V.,
                                        as Guarantor


                                 News T Investments, Inc.,
                                        as Guarantor


                                 News Triangle Finance, Inc.,
                                        as Guarantor


                                 Newscorp Investments,
                                        as Guarantor



                                 Newscorp Overseas Limited,
                                        as Guarantor



                                 Twentieth Century Fox Film Corporation,
                                        as Guarantor


                                 Twentieth Century Fox Home
                                 Entertainment, Inc.,
                                        as Guarantor



                                 By: /s/ Arthur M. Siskind
                                     ----------------------------------------
                                     Name:  Arthur M. Siskind
                                     Title: Senior Executive Vice President,
                                            News America
                                            Incorporated, as Attorney
                                            for the Guarantors

                                      -7-
<PAGE>

                                 The Bank of New York


                                 By:
                                    ---------------------------------
                                    Name:
                                    Title:


     [seal]



Attest:

                                      -8-
<PAGE>

STATE OF NEW YORK                   )
                                    )  :  ss.:
COUNTY OF NEW YORK                  )

     On this 24 day of March, 1999, before me personally appeared Arthur M.
Siskind who acknowledged himself to be a Senior Executive Vice President of News
America Incorporated, and that he, as such Senior Executive Vice President,
being authorized so to do, executed the foregoing instrument for the purposes
therein contained, by signing the name of the corporation by himself as such
Senior Executive Vice President.

     IN WITNESS WHEREOF, I hereunto set my hand and official seal.




Notary Public       Angela D. Weissert


[Notarial Seal]                ANGELA D. WEISSERT
                        Notary Public, State of New York
                                 No. 41-4956083
                           Qualified in Nassau County
                        Commission Expires Sept. 11, 1999

                                      -9-
<PAGE>

STATE OF NEW YORK                   )
                                    )  :  ss.:
COUNTY OF NEW YORK                  )

     On the 25/th/ day of March, 1999, before me personally came Michael Culhane
to me known, who, being by me duly sworn, did depose and say that s/he is a Vice
President of The Bank of New York, the New York banking corporation described in
and which executed the foregoing instrument by authority of the Board of
Directors of said New York banking corporation, and that s/he signed her/his
name thereto by like authority.

     IN WITNESS WHEREOF, I hereunto set my hand and official seal.



Notary Public
                               PETER DEMOS
                         Notary Public, State of New York
                               No. 02DE4983115
[Notarial Seal]             Qualified in Orange County
                          My Commission Expires June 24, 1999

                           /s/ PETER DEMOS

                                      -10-