BINGHAM McCUTCHEN LLP
                               ONE FEDERAL STREET
                          BOSTON, MASSACHUSETTS 02110

                                    December 10, 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.
                                                           ---------

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 has removed the statement referenced by the Staff
             that, for purposes of the Fund's investment policies, equity
             securities include debt convertible to equity securities.

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.

   Response: The Registrant has added disclosure which clarifies that at least
             80% of the net assets of an ETF in which the Fund invests for
             purposes of satisfying the Fund's 80% policy

                                       2





             would be determined to constitute equity securities of large
             companies, based on the ETF's constituent securities.

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.

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


                                       3




             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.

   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.


                                       4




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.

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


                                       5




             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

                                       6



                                                                       Exhibit A
                                                                       ---------

                             Pioneer Series Trust X
                                60 State Street
                          Boston, Massachusetts 02110

                                     December 10, 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:

     In connection with the review by the Staff of the Securities and Exchange
Commission (the "Commission") of Post-Effective Amendment No. 24 to the
Registration Statement on Form N-1A of Pioneer Series Trust X (the
"Registrant"), filed on September 28, 2012 with respect to its series Pioneer
Fundamental Growth Fund, the Registrant acknowledges that, with respect to
filings made by the Registrant with the Commission and reviewed by the Staff:

     (a)  the Registrant is responsible for the adequacy and accuracy of the
          disclosure in the filings;

     (b)  Staff comments or changes to disclosure in response to Staff comments
          in the filings reviewed by the Staff do not foreclose the Commission
          from taking any action with respect to the filings; and

     (c)  the Registrant may not assert Staff comments as a defense in any
          proceeding initiated by the Commission or any person under the federal
          securities laws of the United States.

                                           Sincerely,

                                           Pioneer Series Trust X

                                           By:    /s/ Christopher J. Kelley
                                                  ------------------------------
                                           Name:  Christopher J. Kelley
                                           Title: Secretary