REPORT OF INDEPENDENT REGISTERED PUBLIC
ACCOUNTING FIRM
To the Board of Trustees of MFS Series Trust XVI and the
Shareholders of MFS Prudent Investor Fund:

In planning and performing our audit of the financial
statements of MFS Prudent Investor Fund (the Fund)
as of and for the year ended June 30, 2021, in accordance
with the standards of the Public Company Accounting
Oversight Board (United States), we considered the Funds
internal control over financial reporting, including controls
over safeguarding securities, as a basis for designing our
auditing procedures for the purpose of expressing our opinions
on the financial statements and to comply with the requirements
of Form N-CEN, but not for the purpose of expressing an opinion
on the effectiveness of the Funds internal control over
financial reporting. Accordingly, we express no such opinion.
The management of the Fund is responsible for establishing and
maintaining effective internal control over financial reporting.
In fulfilling this responsibility, estimates and judgments by
management are required to assess the expected benefits and
related costs of controls. A funds internal control over
financial reporting is a process designed to provide reasonable
assurance regarding the reliability of financial reporting and
the preparation of financial statements for external purposes in
accordance with generally accepted accounting principles.
A funds internal control over financial reporting includes
those policies and procedures that (1) pertain to the
maintenance of records that, in reasonable detail,
accurately and fairly reflect the transactions
and dispositions of the assets of the fund;
(2) provide reasonable assurance that transactions are
recorded as necessary to permit preparation of financial
statements in accordance with generally accepted
accounting principles, and that receipts and expenditures
of the fund are being made only in accordance with authorizations
of management of the fund and trustees of the trust; and
(3) provide reasonable assurance regarding prevention or
timely detection of unauthorized acquisition, use, or
disposition of a funds assets that could have a material
effect on the financial statements.


Because of its inherent limitations, internal control over
financial reporting may not prevent or detect misstatements.
Also, projections of any evaluation of effectiveness to future
periods are subject to the risk that controls may become
inadequate because of changes in conditions or that the degree
of compliance with the policies or procedures may deteriorate.
A deficiency in internal control over financial reporting exists
when the design or operation of a control does not allow
management or employees, in the normal course of performing
their assigned functions, to prevent or detect misstatements
on a timely basis. A material weakness is a deficiency,
or a combination of deficiencies, in internal control over
financial reporting, such that there is a reasonable possibility
that a material misstatement of a funds annual or interim
financial statements will not be prevented or detected on a
timely basis. Our consideration of the Funds internal control
over financial reporting was for the limited purpose described
in the first paragraph and would not necessarily disclose all
deficiencies in internal control that might be material
weaknesses under standards established by the Public
Company Accounting Oversight Board (United States). However, we
noted no deficiencies in the Funds internal control over
financial reporting and its operations, including controls for
safeguarding securities, that we consider to be a material
weakness, as defined above, as of June 30, 2021. This report is
intended solely for the information and use of management,
the Board of Trustees of MFS Series Trust XVI and the Securities
and Exchange Commission and is not intended to be and should not
be used by anyone other than these specified parties.
DELOITTE & TOUCHE LLP

Boston, Massachusetts
August 13, 2021