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EXHIBIT 12 B1
CERTIFICATION OF PRINCIPAL EXECUTIVE OFFICER
CERTIFICATIONS
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I, Ronald E. Robison, certify that:
1. I have reviewed this report on Form N-CSR of Morgan Stanley Mortgage
Securities Trust;
2. Based on my knowledge, this report does not contain any untrue statement
of a material fact or omit to state a material fact necessary to make the
statements made, in light of the circumstances under which such
statements were made, not misleading with respect to the period covered
by this report;
3. Based on my knowledge, the financial statements and other financial
information included in this report, fairly present in all material
respects the financial condition, results of operations, changes in net
assets, and cash flows (if the financial statements are required to
include a statement of cash flows) of the registrant as of, and for, the
periods presented in this report;
4. The registrant's other certifying officers and I are responsible for
establishing and maintaining disclosure controls and procedures (as
defined in Rule 30a-3(c) under the Investment Company Act of 1940) and
internal control over financial reporting (as defined in Rule 30a-3(d)
under the Investment Company Act of 1940) for the registrant and have:
a) designed such disclosure controls and procedures, or caused such
disclosure controls and procedures to be designed under our supervision,
to ensure that material information relating to the registrant, including
its consolidated subsidiaries, is made known to us by others within those
entities, particularly during the period in which this report is being
prepared;
b) designed such internal control over financial reporting, or caused such
internal control over financial reporting to be designed under our
supervision, 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;
c) evaluated the effectiveness of the registrant's disclosure controls and
procedures and presented in this report our conclusions about the
effectiveness of the disclosure controls and procedures, as of a date
within 90 days prior to the filing date of this report based on such
evaluation; and
d) disclosed in this report any change in the registrant's internal control
over financial reporting that occurred during the registrant's most
recent fiscal half-year (the registrant's second fiscal half-year in the
case of an annual report) that has materially affected, or is reasonably
likely to materially affect, the registrant's internal control over
financial reporting; and
5. The registrant's other certifying officer(s) and I have disclosed to the
registrant's auditors and the audit committee of the registrant's board
of directors (or persons performing the equivalent functions):
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a) all significant deficiencies and material weaknesses in the design or
operation of internal control over financial reporting which are
reasonably likely to adversely affect the registrant's ability to record,
process, summarize, and report financial information; and
b) any fraud, whether or not material, that involves management or other
employees who have a significant role in the registrant's internal
controls over financial reporting.
Date: June 20, 2006
/s/ Ronald E. Robison
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Ronald E. Robison
Principal Executive Officer
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EXHIBIT 12 B2
CERTIFICATION OF PRINCIPAL FINANCIAL OFFICER
CERTIFICATIONS
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I, Francis Smith, certify that:
1. I have reviewed this report on Form N-CSR of Morgan Stanley Mortgage
Securities Trust;
2. Based on my knowledge, this report does not contain any untrue statement
of a material fact or omit to state a material fact necessary to make the
statements made, in light of the circumstances under which such
statements were made, not misleading with respect to the period covered
by this report;
3. Based on my knowledge, the financial statements and other financial
information included in this report, fairly present in all material
respects the financial condition, results of operations, changes in net
assets, and cash flows (if the financial statements are required to
include a statement of cash flows) of the registrant as of, and for, the
periods presented in this report;
4. The registrant's other certifying officers and I are responsible for
establishing and maintaining disclosure controls and procedures (as
defined in Rule 30a-3(c) under the Investment Company Act of 1940) and
internal control over financial reporting (as defined in Rule 30a-3(d)
under the Investment Company Act of 1940) for the registrant and have:
a) designed such disclosure controls and procedures, or caused such
disclosure controls and procedures to be designed under our supervision,
to ensure that material information relating to the registrant, including
its consolidated subsidiaries, is made known to us by others within those
entities, particularly during the period in which this report is being
prepared;
b) designed such internal control over financial reporting, or caused such
internal control over financial reporting to be designed under our
supervision, 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;
c) evaluated the effectiveness of the registrant's disclosure controls and
procedures and presented in this report our conclusions about the
effectiveness of the disclosure controls and procedures, as of a date
within 90 days prior to the filing date of this report based on such
evaluation; and
d) disclosed in this report any change in the registrant's internal control
over financial reporting that occurred during the registrant's most
recent fiscal half-year (the registrant's second fiscal half-year in the
case of an annual report) that has materially affected, or is reasonably
likely to materially affect, the registrant's internal control over
financial reporting; and
5. The registrant's other certifying officer(s) and I have disclosed to the
registrant's auditors and the audit committee of the registrant's board
of directors (or persons performing the equivalent functions):
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a) all significant deficiencies and material weaknesses in the design or
operation of internal control over financial reporting which are
reasonably likely to adversely affect the registrant's ability to record,
process, summarize, and report financial information; and
b) any fraud, whether or not material, that involves management or other
employees who have a significant role in the registrant's internal
controls over financial reporting.
Date: June 20, 2006
/s/ Francis Smith
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Francis Smith
Principal Financial Officer