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                                                                     Exhibit 4.8

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                       CROWN CASTLE INTERNATIONAL CORP.

                                   As Issuer

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                             SERIES A AND SERIES B

            12 3/4% Senior Subordinated Exchange Debentures due 2010

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                            SUPPLEMENTAL INDENTURE

                         Dated as of December 20, 1999

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  Supplementing the Exchange Indenture dated as of December 21, 1998, between
Crown Castle International Corp., as Issuer, and United States Trust Company of
                            Texas, N.A., as Trustee

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                  UNITED STATES TRUST COMPANY OF TEXAS, N.A.

                                  As Trustee

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   SUPPLEMENTAL INDENTURE dated as of December 20, 1999, between Crown Castle
International Corp., a Delaware corporation (the "Company"), and United States
Trust Company of Texas, N.A. (the "Trustee"), as Trustee under the Exchange
Indenture referred to herein.

   WHEREAS the Company and the Trustee heretofore executed and delivered an
Exchange Indenture dated as of December 21, 1998 (the "Exchange Indenture"), in
respect of the Company's 123/4% Series A Senior Subordinated Exchange
Debentures due 2010 and 123/4% Series B Senior Subordinated Exchange Debentures
due 2010 (collectively, the "Debentures");

   WHEREAS Section 9.02 of the Exchange Indenture provides that the Company and
the Trustee may amend the Exchange Indenture with the consent of the Holders of
at least a majority in aggregate principal amount of the Debentures then
outstanding or, if no Debentures are outstanding, the holders of a majority in
Liquidation Preference of the Senior Exchangeable Preferred Stock then
outstanding;

   WHEREAS the Company desires to amend certain provisions of the Exchange
Indenture, as set forth in Article I hereof;

   WHEREAS there are no Debentures outstanding and holders of a majority in
Liquidation Preference of the Senior Exchangeable Preferred Stock outstanding
have consented to the amendments effected by this Supplemental Indenture; and

   WHEREAS this Supplemental Exchange Indenture has been duly authorized by all
necessary corporate action on the part of the Company.

   NOW, THEREFORE, the Company and the Trustee agree as follows for the equal
and ratable benefit of the Holders of the Debentures:

                                   ARTICLE I

                                  Amendments

   SECTION 1.01.  Amendments to Section 4.09. (a) Modification of Incurrence of
Indebtedness and Issuance of Preferred Stock Covenant.  The first paragraph of
Section 4.09 (Incurrence of Indebtedness and Issuance of Preferred Stock) of
the Exchange Indenture is hereby deleted in its entirety and replaced with the
following:

      "The Company shall not, and shall not permit any of its Restricted
   Subsidiaries to, directly or indirectly, create, incur, issue, assume,
   guarantee or otherwise become directly or indirectly liable, contingently or
   otherwise, with respect to (collectively, "incur") any Indebtedness
   (including Acquired Debt) and the Company will not issue any Disqualified
   Stock and will not permit any of its Restricted Subsidiaries to issue any
   shares of preferred stock; provided that the Company may incur Indebtedness
   (including Acquired Debt) or issue shares of Disqualified Stock and the
   Company's Restricted Subsidiaries may incur Indebtedness (including Acquired
   Debt) or issue preferred stock if, in each case, the Company's Debt to
   Adjusted Consolidated Cash Flow Ratio at the time of incurrence of such
   Indebtedness or the issuance of such Disqualified Stock or preferred stock,
   after giving pro forma effect to such incurrence or issuance as of such date
   and to the use of proceeds from such incurrence or issuance as if the same
   had occurred at the beginning of the most recently ended four full fiscal
   quarter period of the Company for which internal financial statements are
   available, would have been no greater than 7.5 to 1.".

   (b)  Modification of Formula for Credit Facilities Indebtedness.  The first
clause of the definition of "Permitted Debt" contained in Section 4.09
(Incurrence of Indebtedness and Issuance of Preferred Stock) of the Exchange
Indenture is hereby deleted in its entirety and replaced with the following:

      "(1)  the incurrence by the Company or any of its Restricted Subsidiaries
   of Indebtedness under Credit Facilities in an aggregate principal amount
   (with letters of credit being deemed to have a principal amount

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   equal to the maximum potential liability of the Company and its Restricted
   Subsidiaries thereunder) at any one time outstanding not to exceed (x) in
   the case of an incurrence on or before December 31, 2000, the greater of (i)
   $500.0 million or (ii) an amount equal to the Adjusted Consolidated Cash
   Flow of the Company at the time of such incurrence multiplied by 8; (y) in
   the case of an incurrence during the period beginning on and including
   January 1, 2001 to and including December 31, 2001, an amount equal to the
   Adjusted Consolidated Cash Flow of the Company at the time of such
   incurrence multiplied by 7.5; and (z) in the case of an incurrence after
   December 31, 2001, an amount equal to the Adjusted Consolidated Cash Flow of
   the Company at the time of such incurrence multiplied by 6.5;".

   SECTION 1.02.  Amendment to Section 4.17.  The following sentence is hereby
added to the end of Section 4.17 (Limitation on Issuances and Sales of Capital
Stock of Restricted Subsidiaries) of the Exchange Indenture:

      "Notwithstanding the foregoing, the issuance or sale of shares of Capital
   Stock of any Restricted Subsidiary of the Company will not violate the
   provisions of the immediately preceding sentence if such shares are issued
   or sold in connection with (x) the formation or capitalization of a
   Restricted Subsidiary or (y) a single transaction or a series of
   substantially contemporaneous transactions whereby such Restricted
   Subsidiary becomes a Restricted Subsidiary of the Company by reason of the
   acquisition of securities or assets from another Person.".

   SECTION 1.03.  Trustee's Acceptance.  The Trustee hereby accepts this
Supplemental Indenture and agrees to perform the same under the terms and
conditions set forth in the Exchange Indenture.

                                  ARTICLE II

                                 Miscellaneous

   SECTION 2.01.  Interpretation.  Upon execution and delivery of this
Supplemental Indenture, the Exchange Indenture shall be modified and amended in
accordance with this Supplemental Indenture, and all the terms and conditions
of both shall be read together as though they constitute one instrument, except
that, in case of conflict, the provisions of this Supplemental Indenture shall
control. The Exchange Indenture, as modified and amended by this Supplemental
Indenture, is hereby ratified and confirmed in all respects and shall bind
every Holder of Debentures. In case of conflict between the terms and
conditions contained in the Debentures and those contained in the Exchange
Indenture, as modified and amended by this Supplemental Indenture, the
provisions of the Exchange Indenture, as modified and amended by this
Supplemental Indenture, shall control.

   SECTION 2.02.  Conflict with Trust Indenture Act.  If any provision of this
Supplemental Indenture limits, qualifies or conflicts with any provision of the
TIA that is required under the TIA to be part of and govern any provision of
this Supplemental Indenture, the provision of the TIA shall control. If any
provision of this Supplemental Indenture modifies or excludes any provision of
the TIA that may be so modified or excluded, the provision of the TIA shall be
deemed to apply to the Exchange Indenture as so modified or to be excluded by
this Supplemental Indenture, as the case may be.

   SECTION 2.03.  Severability.  In case any provision in this Supplemental
Indenture shall be invalid, illegal or unenforceable, the validity, legality
and enforceability of the remaining provisions shall not in any way be affected
or impaired thereby.

   SECTION 2.04.  Terms Defined in the Exchange Indenture.  All capitalized
terms not otherwise defined herein shall have the meanings ascribed to them in
the Exchange Indenture.

   SECTION 2.05.  Headings.  The Article and Section headings of this
Supplemental Indenture have been inserted for convenience of reference only,
are not to be considered a part of this Supplemental Indenture and shall in no
way modify or restrict any of the terms or provisions hereof.

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   SECTION 2.06.  Benefits of Supplemental Indenture, etc.  Nothing in this
Supplemental Indenture or the Debentures, express or implied, shall give to any
Person, other than the parties hereto and thereto and their successors
hereunder and thereunder and the Holders of the Debentures, any benefit of any
legal or equitable right, remedy or claim under the Exchange Indenture, this
Supplemental Indenture or the Debentures.

   SECTION 2.07.  Successors.  All agreements of the Company in this
Supplemental Indenture shall bind its successors. All agreements of the Trustee
in this Supplemental Indenture shall bind its successors.

   SECTION 2.08.  Trustee Not Responsible for Recitals.  The recitals contained
herein shall be taken as the statements of the Company and the Trustee assumes
no responsibility for their correctness.

   SECTION 2.09.  Certain Duties and Responsibilities of the Trustee.  In
entering into this Supplemental Indenture, the Trustee shall be entitled to the
benefit of every provision of the Exchange Indenture relating to the conduct or
affecting the liability or affording protection to the Trustee, whether or not
elsewhere herein so provided.

   SECTION 2.10.  Governing Law.  This Supplemental Indenture shall be governed
by, and construed in accordance with, the laws of the State of New York but
without giving effect to applicable principles of conflicts of law to the
extent that the application of the laws of another jurisdiction would be
required thereby.

   SECTION 2.11.  Counterpart Originals.  The parties may sign any number of
copies of this Supplemental Indenture. Each signed copy shall be an original,
but all of them together represent the same agreement.

   IN WITNESS WHEREOF, each party hereto has caused this Supplemental Indenture
to be signed by its officer thereunto duly authorized as of the date first
written above.

                                              CROWN CASTLE INTERNATIONAL CORP.,

                                              by  -----------------------------
                                                  Name: W. Benjamin Moreland
                                                  Title: Senior Vice President
                                                    and Treasurer

                                              UNITED STATES TRUST COMPANY OF
                                                TEXAS, N.A., as Trustee,

                                              by  -----------------------------
                                                  Name:
                                                  Title: