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                                                                     Exhibit 4.2

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                                     UBS AG

                                       TO

                      U.S. BANK TRUST NATIONAL ASSOCIATION
                                     Trustee

                             -----------------------



                          FIRST SUPPLEMENTAL INDENTURE

                          Dated as of February 28, 2006

                          Supplement to Indenture Dated
                             as of November 21, 2000


                             -----------------------

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      FIRST SUPPLEMENTAL INDENTURE, dated as of February 28, 2006 to Indenture
dated as of November 21, 2000 (the "INDENTURE") between UBS AG, a corporation
duly organized and existing under the laws of Switzerland (herein called the
"COMPANY"), having its principal office at Bahnhofstrasse 45, Zurich,
Switzerland, and U.S. BANK TRUST NATIONAL ASSOCIATION, a national banking
association duly organized and existing under the laws of the United States, as
Trustee (the "TRUSTEE").

                            RECITALS OF THE COMPANY:

      WHEREAS, Section 901(5) of the Indenture provides that, without the
consent of any Holders, the Company, when authorized by a Board Resolution, and
the Trustee, at any time and from time to time, may enter into one or more
indentures supplemental to the Indenture to change any of the provisions of the
Indenture in respect of one or more series of Securities created after the
execution of such supplemental indenture ("AFFECTED SECURITIES");

      WHEREAS, the Company desires to modify certain provisions of the Indenture
relating to the notice of redemption by the Company;

      WHEREAS, the entry into this First Supplemental Indenture by the parties
hereto is in all respects authorized by the provisions of the Indenture; and

      WHEREAS, this First Supplemental Indenture is authorized by the Board
Resolution dated 18 December 2002, and all things necessary to make this First
Supplemental Indenture a valid agreement of the Company and the Trustee
according to its terms have been done;

      NOW, THEREFORE, THIS FIRST SUPPLEMENTAL INDENTURE WITNESSETH:

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



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                                    ARTICLE 1
                                   Amendments

      Section 1.01. Notice on Election to Redeem; Notice to Trustee. Section
1102 of the Indenture is hereby amended, with respect to any Affected
Securities, by deleting the phrase "at least 60 days" and replacing it with the
phrase "at least 5 Business Days".

      Section 1.02. Notice of Redemption. Section 1104 of the Indenture is
hereby amended, with respect to any Affected Securities, by deleting the phrase
"not less than 10" and replacing it with the phrase "not less than 5 Business
Days".

                                   ARTICLE 2
                            Miscellaneous Provisions

      Section 2.01. Other Terms of Indenture. Except insofar as herein
otherwise expressly provided, all provisions, terms and conditions of the
Indenture are in all respects ratified and confirmed and shall remain in full
force and effect.

      Section 2.02. Terms Defined. All terms defined elsewhere in the Indenture
shall have the same meanings when used herein.

      Section 2.03. Separability Clause. In case any provision in this First
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. Governing Law. This First Supplemental Indenture shall be
governed by and construed in accordance with the law of the State of New York.

      Section 2.05. Counterparts. This First Supplemental Indenture may be
executed in any number of counterparts, each of which shall be an original; but
such counterparts shall together constitute but one and the same instrument.


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

                                          UBS AG

                                          By: /s/ BRYAN MURTAGH
                                              -------------------
                                          Name:
                                          Title:

                                          UBS AG

                                          By: /s/ DAVID KELLY
                                              --------------------------
                                          Name:  David Kelly
                                          Title: Managing Director and
                                                 Counsel Region
                                                 Americas Legal








                                          U.S. BANK TRUST NATIONAL
                                          ASSOCIATION

                                          By: /s/ DAVID J. KOLIBACHUK
                                              --------------------------
                                          Name:  David J. Kolibachuk
                                          Title: Vice President



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