Item 12(b)
                                                   Exhibit 99.906CERT
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Certification Pursuant to Section 906 of the Sarbanes-Oxley Act of
2002 (subsections (a) and (b) of section 1350, chapter 63 of title 18,
United States Code)

In connection with the attached Report on Form N-CSR of Thornburg
Investment Trust in respect of the following Thornburg Funds: Value
Fund, International Value Fund, Core Growth Fund and Investment Income
Builder Fund, (hereafter referred to as the "Funds") to be filed with
the Securities and Exchange Commission (the "Report"), each of the
undersigned officers of the Trust does hereby certify pursuant to
Section 906 of the Sarbanes-Oxley Act of 2002 that, to the best of
such officer's knowledge:

        1. The Report fully complies with the requirements of 13(a) or
15(d) of the Securities Exchange Act of 1934; and

        2. The information contained in the Report fairly presents, in
all material respects, the financial condition and results of
operations of Thornburg Investment Trust, in respect of the Funds as
of, and for, the periods presented in the Report.

Date: June 23, 2008
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      /s/ BRIAN J. MCMAHON
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      Brian J. McMahon
      President and principal executive officer

Date: June 23, 2008
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      /s/ GEORGE T. STRICKLAND
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      George T. Strickland
      Treasurer and principal financial officer

A signed original of this written statement required by Section 906,
or other document authenticating, acknowledging, or otherwise adopting
the signature that appears in typed form within the electronic version
of this written statement required by Section 906, has been provided
to the Trust and will be retained by the Trust and furnished to the
Securities and Exchange Commission or its staff upon request.