BINGHAM McCUTCHEN LLP
                              ONE FEDERAL STREET
                          BOSTON, MASSACHUSETTS 02110

                                                     December 3, 2012

VIA EDGAR
--- -----

Securities and Exchange Commission
Division of Investment Management
100 F Street, NE
Washington, D.C. 20549

Re:Pioneer Series Trust X
   Registration Statement on Form N-1A
   (File Nos. 333-89354; 811-21108)

Ladies and Gentlemen:

   This letter is to respond to comments we received from Mr. Dominic Minore of
the Staff of the Division of Investment Management of the Securities and
Exchange Commission (the "Commission") regarding Post-Effective Amendment
No. 24 to the Registration Statement on Form N-1A for Pioneer Series Trust X
(the "Registrant") relating to Pioneer Fundamental Growth Fund (the "Fund").
Following are the Staff's comments and the Registrant's responses thereto:

I. General Comment

1. Comment:   The Staff asked that the Registrant provide a letter to the
              Commission that includes certain "Tandy" acknowledgments with the
              Registrant's response to the Staff's comments.

   Response:  A Tandy representation letter executed in connection with the
              filing of this response is attached hereto as Exhibit A.
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2. Comment:   The Staff requested that the Registrant revise the disclosure in
              the Fund's prospectus and statement of additional information or
              otherwise respond to the applicable Staff comments on
              Post-Effective Amendment No. 21 to the Registration Statement of
              Pioneer Series Trust VII provided by Mr. Minore on November 9,
              2012.

   Response:  The Registrant has revised the disclosure in the Fund's
              prospectus and statement of additional information or otherwise
              responded herein to the applicable Staff comments on
              Post-Effective Amendment No. 21 to the Registration Statement of
              Pioneer Series Trust VII provided by Mr. Minore on November 9,
              2012.



II. Comments Applicable to the Prospectus

A. Fees and Expenses of the Fund

1. Comment:   The Staff noted that Form N-1A permits a fee waiver arrangement
              to be shown in the Fee Table only if the arrangement will reduce
              Fund operating expenses for no less than one year from the
              effective date of the Fund's Registration Statement. The Staff
              requested that Pioneer extend the fee waiver arrangement or
              remove it from the Fee Table.

   Response:  The Registrant has revised the disclosure to indicate that the
              fee waiver arrangement is in effect through August 1, 2014.

2. Comment:   The Staff requested that the Registrant confirm that the Fund
              does not intend to invest in other investment companies to the
              extent that Acquired Fund Fees and Expenses will represent 0.01%
              or more of the Fund's expenses during the next twelve months.

   Response:  The Registrant confirms that the Fund does not intend to invest
              in other investment companies to the extent that Acquired Fund
              Fees and Expenses will represent 0.01% or more of the Fund's
              expenses during the next twelve months.

3. Comment:   The Staff requested that the Registrant file the Expense Limit
              Agreement with respect to the Fund referenced in Footnote 2 to
              the Fee Table as an exhibit to the next Post-Effective Amendment
              to the Registrant's Registration Statement relating to the Fund.

   Response:  The Registrant confirms that it filed the Expense Limit Agreement
              with respect to the Fund referenced in Footnote 2 to the Fee
              Table as an exhibit to Post-Effective Amendment No. 25 to the
              Registrant's Registration Statement relating to the Fund.

B. Principal Investment Strategies

1. Comment:   The Staff requested that the Registrant revise the disclosure to
              clarify the extent to which the Fund may invest across all
              industries and market sectors.

   Response:  The Registrant has added disclosure to clarify that the Fund may
              invest in securities of issuers across all industries and market
              sectors.

2. Comment:   The Staff requested that the Registrant clarify in its response
              to the Staff's comments that the Fund will consider investments
              in debt convertible to equity securities as equity securities for
              purposes of the Fund's investment policies only to the extent
              such convertible debt is "immediately" convertible to equity.

   Response:  The Registrant respectfully notes that a convertible debt
              security that is not "immediately" convertible to equity
              nevertheless may exhibit the characteristics of an equity
              security, depending on the relationship of the conversion price
              to the market value of the underlying security. Accordingly,
              investments in convertible debt securities will be considered
              equity securities for purposes of the Fund's investment policies
              even if such securities are not immediately convertible to equity.

3. Comment:   The Staff requested that the Registrant clarify that, to the
              extent that it counts investments in ETFs towards the Fund's 80%
              test, such ETFs will have an 80% test that mirrors the Fund's 80%
              test.

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   Response:  The Registrant has added disclosure which clarifies that for
              purposes of the fund's investment policies, an investment in an
              ETF will be considered to be an investment in an equity security
              of a large company if the ETF is based on an index, the
              characteristics of which indicate that it is comprised of equity
              securities of large companies.

4. Comment:   With respect to the Fund's investment in junk bonds, the Staff
              requested that the Registrant clarify how low in the credit
              spectrum the Fund may invest (for example, by indicating if the
              Fund is permitted to invest in securities of issuers that are in
              default).

   Response:  The Registrant has added disclosure to clarify that the Fund may
              invest in securities of issuers that are in default.

5. Comment:   The Staff requested that the Registrant confirm in its response
              to the Staff's comments that the disclosure in the prospectus
              regarding the Fund's use of derivatives addresses the
              considerations noted in the letter from Barry D. Miller,
              Associate Director, Division of Investment Management, SEC to
              Karrie McMillan, General Counsel, ICI (July 30, 2010) regarding
              derivatives related disclosure by investment companies.

   Response:  The Registrant confirms that the disclosure in the prospectus
              regarding the Fund's use of derivatives addresses the
              considerations noted in the letter from the Division of
              Investment Management to the Investment Company Institute
              regarding derivatives-related disclosure by investment companies.

C. Principal Risks of Investing in the Fund

1. Comment:   The Staff requested that the Registrant consider adding to the
              disclosure regarding "Risks of Non-U.S. Investments" in the
              summary section of the prospectus that less information about
              non-U.S. issuers or markets may be available and that withholding
              and other non-U.S. taxes may decrease the Fund's return.

   Response:  The Registrant has added disclosure to Risks of Non-U.S.
              Investments" in the summary section of the prospectus noting that
              lack of information and less market regulation may affect the
              value of securities of non-U.S. issuers and that withholding and
              other non-U.S. taxes may decrease the Fund's return.

2. Comment:   The Staff noted that the Registrant's disclosure regarding
              "Leveraging Risk" states that "the value of your investment may
              be more volatile and other risks tend to be compounded if the
              Fund borrows or uses derivatives or other investments, such as
              ETFs, that have embedded leverage." The Staff asked if the Fund's
              use of borrowing or its investments in ETFs are principal
              investment strategies of the Fund and, if so, to expand
              the principal strategy and risk disclosure with respect to the
              Fund's use of borrowing and investments in ETFs.

   Response:  The Registrant confirms that neither the Fund's use of borrowing
              nor its investments in ETFs are principal investment strategies
              of the Fund. The Registrant notes that the references to
              borrowing and to ETFs are included in the disclosure referenced
              by the Staff as examples of investment practices that may result
              in leverage rather than as references to specific principal
              investment strategies. Accordingly, the Registrant respectfully
              submits that no additional disclosure is required.


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3. Comment:   The Staff requested that the Registrant revise the last sentence
              of the "Principal Investment Risks" section to state that "there
              are many other non-principal factors that could adversely affect
              your investment and that could prevent the Fund from achieving
              its goals" or expand the disclosure in this section to address
              any other principal risks of investing in the Fund.

   Response:  The Registrant notes that it currently discloses the principal
              risks of investing in the Fund in the Fund summary. The
              Registrant further notes that the disclosure referenced by the
              Staff does not suggest that the "other factors" referred to in
              such disclosure are principal risks. The Registrant respectfully
              submits that the additional disclosure suggested by the Staff
              would not be helpful to investors, and, accordingly, that no
              additional disclosure is required.

D. Tax Information

1. Comment:   The Staff noted that the Registrant states that "you normally
              will have to pay income taxes on the dividends and capital gain
              distributions you receive from the Fund, unless you are investing
              through a tax-deferred account, such as a 401(k) plan or
              individual retirement account." The Staff requested that the
              Registrant revise the disclosure to clarify that an investor
              normally will pay taxes on dividends and capital gain
              distributions when such distributions are withdrawn from such
              tax-deferred accounts.

   Response:  The Registrant has revised the disclosure referenced by the Staff
              to state that the Fund intends to make distributions that may be
              taxed as ordinary income or capital gains, tracking the language
              provided in Item 7 of Form N-1A.

E. More on the Fund's Investment Objectives and Strategies

1. Comment:   The Staff noted that the Registrant states that the Fund's
              investment objective may be changed without shareholder approval
              and that the Fund will provide notice prior to implementing any
              change to its investment objective. The Staff requested that the
              Registrant disclose the amount of advance notice the Fund will
              provide to shareholders before changing its investment objective.

   Response:  The Registrant has added disclosure noting that the Fund will
              provide at least 30 days' notice prior to implementing any change
              to its investment objective.

2. Comment:   The Staff requested that the Registrant revise the section titled
              "Non-U.S. Investments" to restate the limitations on the Fund's
              investments in securities of non-U.S. issuers and emerging
              markets issuers.

   Response:  The Registrant has revised the section titled "Non-U.S.
              Investments" to restate the limitations on the Fund's investments
              in securities of non-U.S. issuers and emerging markets issuers.

3. Comment:   The Staff requested that the Registrant revise the disclosure
              regarding "Cash Management and Temporary Investments" to indicate
              that the Fund may be prevented from achieving its investment
              objective when the Fund takes temporary defensive positions.

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   Response:  The Registrant has revised the disclosure regarding "Cash
              Management and Temporary Investments" to indicate that the Fund
              may be prevented from achieving its investment objective when the
              Fund takes temporary defensive positions.

F. Investment Adviser

1. Comment:   The Staff requested that the Registrant delete the statement
              that, to the extent that the Commission adopts a rule that would
              supersede the Fund's manager-of-managers exemptive order, or
              would provide greater flexibility than the order, Pioneer and the
              Fund intend to rely on such rule. The Staff noted that such
              statement is speculative and that the Commission has no current
              plans to adopt such a rule.

   Response:  The Registrant has deleted the disclosure referenced by the Staff.

G. Concentration Policy

1. Comment:   The Staff noted that the Registrant states that "except as
              permitted by exemptive or other relief or permission from the
              SEC, SEC Staff or other authority of competent jurisdiction, the
              Fund may not make any investment if, as a result, the Fund's
              investments will be concentrated in any one industry." The Staff
              noted that, in its view, the Fund's formulation of its
              concentration policy reserves the Fund's freedom of action to
              concentrate in an industry, and that reserving the right to
              concentrate in a particular industry or group of industries
              without limitation is prohibited by Section 8(b)(1) of the 1940
              Act. The Staff requested that the Registrant revise the Fund's
              concentration policy so as not to reserve the Fund's freedom of
              action to concentrate in an industry.

   Response:  The Registrant acknowledges the Staff's comment. The Registrant
              notes that, as currently formulated, the concentration policy
              provides that the Fund may not make any investment if, as a
              result, the Fund's investments will be concentrated in any
              industry unless the SEC, SEC Staff or other authority of
              competent jurisdiction permits the Fund to do so. Thus, it is the
              Registrant's understanding that the Fund does not itself have the
              freedom to make any such investment because the SEC, SEC Staff or
              other authority must take affirmative action to permit such
              investment. Accordingly, the Registrant respectfully submits that
              no change to the Fund's concentration policy is required.
              Furthermore, the Registrant notes that the Fund may not change
              its concentration policy without shareholder approval, and that
              the current formulation of the Fund's concentration policy was
              approved by the Fund's shareholders at a meeting held on May 20,
              2008. The Fund also notes that the Staff reviewed the current
              formulation of the Fund's concentration policy prior to the
              submission of the policy to shareholders for approval.

2. Comment:   The Staff requested that the Registrant revise the disclosure
              regarding tax-exempt funds that invest 80% of their assets in
              tax-exempt securities to clarify that such funds will
              characterize investments in revenue bonds by the types of
              projects from which such revenue is derived.

   Response:  The Registrant has revised the disclosure regarding tax-exempt
              funds that invest 80% of their assets in tax-exempt securities to
              clarify that such funds characterize investments in securities
              the interest upon which is paid from revenue of similar type
              projects by the type or types of projects.

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3. Comment:   The Staff noted that the Registrant states that "when identifying
              industries for purposes of its concentration policy, the Fund may
              rely upon available industry classifications. As of the date of
              the SAI, the Fund relies on the MSCI Global Industry
              Classification Standard (GICS) classifications, or with respect
              to certain securities, another third-party classification system.
              Exchange-traded funds may be classified based on the underlying
              securities." The Staff requested that the Registrant specify the
              "certain securities" and the "third-party classification system"
              that will be used, and also specify the classification system
              that will be used on the underlying securities of exchange-traded
              funds.

   Response:  The Registrant has revised the disclosure referenced by the Staff
              to state that "as of the date of the SAI, the Fund relies
              primarily on the MSCI Global Industry Classification Standard
              (GICS) classifications, and, with respect to securities for which
              no industry classification under GICS is available or for which
              the GICS classification is determined not to be appropriate, the
              Fund may use industry classifications published by another
              source, which, as of the date of the SAI, is Bloomberg L.P. As of
              the date of the SAI, the Fund's adviser may assign an industry
              classification for an exchange-traded fund in which the Fund
              invests based on the constituents of the index on which the
              exchange-traded fund is based. The Fund may change any source
              used for determining industry classifications without shareholder
              approval."

   Please call the undersigned at (617) 951-8458 or Toby Serkin at
(617) 951-8760 with any questions.

                                          Sincerely,

                                          /s/Jeremy Kantrowitz
                                          --------------------
                                          Jeremy Kantrowitz

cc: Terrence J. Cullen
    Christopher J. Kelley
    Roger P. Joseph
    Toby R. Serkin

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