<PAGE> PAGE 1
000 A000000 01/31/98
000 C000000 0000783740
000 D000000 N
000 E000000 NF
000 F000000 Y
000 G000000 N
000 H000000 N
000 I000000 3.0.a
000 J000000 A
001 A000000 MFS SERIES TRUST X
001 B000000 811-4492
001 C000000 6179545000
002 A000000 500 BOYLSTON STREET
002 B000000 BOSTON
002 C000000 MA
002 D010000 02116
003 000000 N
004 000000 N
005 000000 N
006 000000 N
007 A000000 Y
007 B000000 1
007 C010100 1
007 C020100 MFS GOVERNMENT MORTGAGE FUND
007 C030100 N
007 C010200 2
007 C010300 3
007 C010400 4
007 C010500 5
007 C010600 6
007 C010700 7
007 C010800 8
007 C010900 9
007 C011000 10
020 C000001 0
020 C000002 0
020 C000003 0
020 C000004 0
020 C000005 0
020 C000006 0
020 C000007 0
020 C000008 0
020 C000009 0
020 C000010 0
021 000000 0
022 A000001 GOLDMAN SACHS & CO.
022 B000001 13-5108880
022 C000001 4355093
022 D000001 9372
022 A000002 MERRILL LYNCH
022 B000002 13-5674085
<PAGE> PAGE 2
022 C000002 82060
022 D000002 53009
022 A000003 MORGAN STANLEY AND CO, INC
022 B000003 13-2655998
022 C000003 65439
022 D000003 29497
022 A000004 SOLOMON BROTHERS INC
022 B000004 13-3082694
022 C000004 20022
022 D000004 31050
022 A000005 LEHMAN BROTHERS INC
022 B000005 13-2518466
022 C000005 0
022 D000005 38229
022 A000006 BEAR STERNS SECURITIES
022 B000006 13-3299429
022 C000006 0
022 D000006 36493
022 A000007 UBS SECURITIES
022 B000007 13-2932996
022 C000007 0
022 D000007 23840
022 A000008 FIRST ALBANY CORP
022 B000008 00-0000000
022 C000008 0
022 D000008 14869
022 A000009 CS FIRST BOSTON
022 B000009 13-5659485
022 C000009 10438
022 D000009 0
022 A000010 CJ LAWRENCE
022 B000010 00-0000000
022 C000010 0
022 D000010 3152
023 C000000 4533051
023 D000000 241441
026 A000000 Y
026 B000000 Y
026 C000000 N
026 D000000 Y
026 E000000 N
026 F000000 N
026 G010000 N
026 G020000 N
026 H000000 N
027 000000 Y
077 A000000 N
078 000000 N
086 A010000 0
086 A020000 0
086 B010000 0
<PAGE> PAGE 3
086 B020000 0
086 C010000 0
086 C020000 0
086 D010000 0
086 D020000 0
086 E010000 0
086 E020000 0
086 F010000 0
086 F020000 0
008 A000101 MASSACHUSETTS FINANCIAL SERVICES COMPANY
008 B000101 A
008 C000101 801-17352
008 D010101 BOSTON
008 D020101 MA
008 D030101 02116
010 A000101 MASSACHUSETTS FINANCIAL SERVICES COMPANY
010 B000101 701-17352
010 C010101 BOSTON
010 C020101 MA
010 C030101 02116
011 A000101 MFS FUND DISTRIBUTORS, INC.
011 B000101 8-27864
011 C010101 BOSTON
011 C020101 MA
011 C030101 02116
012 A000101 MFS SERVICE CENTER, INC.
012 B000101 84-1316
012 C010101 BOSTON
012 C020101 MA
012 C030101 02116
013 A000101 DELOITTE & TOUCHE LLP
013 B010101 BOSTON
013 B020101 MA
013 B030101 02110
015 A000101 STATE STREET BANK AND TRUST COMPANY
015 B000101 C
015 C010101 BOSTON
015 C020101 MA
015 C030101 02110
015 E010101 X
015 A000102 STATE STREET BANK AND TRUST COMPANY
015 B000102 S
015 C010102 BOSTON
015 C020102 MA
015 C030102 02110
015 E040102 X
015 A000103 THE BANK OF NEW YORK
015 B000103 S
015 C010103 NEW YORK
015 C020103 NY
015 C030103 10286
<PAGE> PAGE 4
015 E010103 X
018 000100 Y
019 A000100 Y
019 B000100 52
019 C000100 MASSFINSER
024 000100 Y
025 A000101 GOLDMAN SACHS
025 B000101 13-5108880
025 C000101 D
025 D000101 39885
025 D000102 0
025 D000103 0
025 D000104 0
025 D000105 0
025 D000106 0
025 D000107 0
025 D000108 0
028 A010100 2204
028 A020100 2312
028 A030100 0
028 A040100 16675
028 B010100 2412
028 B020100 2308
028 B030100 0
028 B040100 15782
028 C010100 1693
028 C020100 2280
028 C030100 0
028 C040100 15437
028 D010100 2806
028 D020100 2240
028 D030100 0
028 D040100 13835
028 E010100 2289
028 E020100 2232
028 E030100 0
028 E040100 17208
028 F010100 87402
028 F020100 2190
028 F030100 0
028 F040100 98141
028 G010100 98806
028 G020100 13562
028 G030100 0
028 G040100 177078
028 H000100 94104
029 000100 Y
030 A000100 78
030 B000100 4.75
030 C000100 0.00
031 A000100 12
<PAGE> PAGE 5
031 B000100 0
032 000100 66
033 000100 0
034 000100 Y
035 000100 63
036 A000100 N
036 B000100 0
037 000100 N
038 000100 0
039 000100 N
040 000100 Y
041 000100 Y
042 A000100 0
042 B000100 0
042 C000100 46
042 D000100 54
042 E000100 0
042 F000100 0
042 G000100 0
042 H000100 0
043 000100 2165
044 000100 0
045 000100 Y
046 000100 N
047 000100 Y
048 000100 0.450
048 A010100 0
048 A020100 0.000
048 B010100 0
048 B020100 0.000
048 C010100 0
048 C020100 0.000
048 D010100 0
048 D020100 0.000
048 E010100 0
048 E020100 0.000
048 F010100 0
048 F020100 0.000
048 G010100 0
048 G020100 0.000
048 H010100 0
048 H020100 0.000
048 I010100 0
048 I020100 0.000
048 J010100 0
048 J020100 0.000
048 K010100 0
048 K020100 0.000
049 000100 N
050 000100 N
051 000100 N
<PAGE> PAGE 6
052 000100 N
053 A000100 Y
053 B000100 N
053 C000100 Y
054 A000100 Y
054 B000100 Y
054 C000100 N
054 D000100 N
054 E000100 N
054 F000100 N
054 G000100 N
054 H000100 Y
054 I000100 N
054 J000100 Y
054 K000100 N
054 L000100 N
054 M000100 Y
054 N000100 N
054 O000100 N
055 A000100 Y
055 B000100 N
056 000100 Y
057 000100 N
058 A000100 N
059 000100 Y
060 A000100 Y
060 B000100 Y
061 000100 1000
062 A000100 Y
062 B000100 0.0
062 C000100 0.0
062 D000100 4.6
062 E000100 0.0
062 F000100 0.0
062 G000100 0.0
062 H000100 0.0
062 I000100 0.0
062 J000100 0.0
062 K000100 0.0
062 L000100 0.0
062 M000100 8.6
062 N000100 85.9
062 O000100 0.0
062 P000100 0.0
062 Q000100 0.2
062 R000100 0.0
063 A000100 0
063 B000100 8.2
064 A000100 Y
064 B000100 N
066 A000100 N
<PAGE> PAGE 7
067 000100 N
068 A000100 N
068 B000100 N
069 000100 N
070 A010100 Y
070 A020100 Y
070 B010100 N
070 B020100 N
070 C010100 Y
070 C020100 N
070 D010100 N
070 D020100 N
070 E010100 Y
070 E020100 Y
070 F010100 N
070 F020100 N
070 G010100 Y
070 G020100 N
070 H010100 N
070 H020100 N
070 I010100 N
070 I020100 N
070 J010100 Y
070 J020100 N
070 K010100 Y
070 K020100 N
070 L010100 N
070 L020100 N
070 M010100 N
070 M020100 N
070 N010100 Y
070 N020100 N
070 O010100 Y
070 O020100 N
070 P010100 Y
070 P020100 N
070 Q010100 N
070 Q020100 N
070 R010100 Y
070 R020100 N
071 A000100 187062
071 B000100 295817
071 C000100 863848
071 D000100 22
072 A000100 6
072 B000100 33902
072 C000100 0
072 D000100 0
072 E000100 0
072 F000100 2048
072 G000100 61
<PAGE> PAGE 8
072 H000100 0
072 I000100 578
072 J000100 150
072 K000100 91
072 L000100 19
072 M000100 37
072 N000100 0
072 O000100 0
072 P000100 0
072 Q000100 0
072 R000100 16
072 S000100 3
072 T000100 2140
072 U000100 0
072 V000100 0
072 W000100 289
072 X000100 5432
072 Y000100 134
072 Z000100 28604
072AA000100 0
072BB000100 1526
072CC010100 2668
072CC020100 0
072DD010100 20496
072DD020100 7603
072EE000100 0
073 A010100 0.2200
073 A020100 0.1900
073 B000100 0.0000
073 C000100 0.0000
074 A000100 872
074 B000100 39885
074 C000100 0
074 D000100 833348
074 E000100 0
074 F000100 0
074 G000100 0
074 H000100 0
074 I000100 0
074 J000100 0
074 K000100 0
074 L000100 40450
074 M000100 9
074 N000100 914565
074 O000100 33313
074 P000100 229
074 Q000100 0
074 R010100 0
074 R020100 0
074 R030100 0
074 R040100 1340
<PAGE> PAGE 9
074 S000100 0
074 T000100 879683
074 U010100 104411
074 U020100 25838
074 V010100 6.75
074 V020100 6.76
074 W000100 0.0000
074 X000100 44581
074 Y000100 0
075 A000100 0
075 B000100 903025
076 000100 0.00
080 A000100 NATIONAL UNION INSURANCE
080 C000100 75000
081 A000100 Y
081 B000100 106
082 A000100 Y
082 B000100 100
083 A000100 N
083 B000100 0
084 A000100 N
084 B000100 0
085 A000100 Y
085 B000100 N
SIGNATURE ELLEN MOYNIHAN
TITLE ASSISTANT TREASURER
<TABLE> <S> <C>
<ARTICLE> 6
<CIK> 0000783740
<NAME> MFS SERIES TRUST X
<SERIES>
<NUMBER> 011
<NAME> MFS GOVERNMENT MORTGAGE FUND CLASS A
<MULTIPLIER> 1
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> JUL-31-1998
<PERIOD-START> AUG-01-1997
<PERIOD-END> JAN-01-1998
<INVESTMENTS-AT-COST> 856548560
<INVESTMENTS-AT-VALUE> 873233419
<RECEIVABLES> 40450382
<ASSETS-OTHER> 9102
<OTHER-ITEMS-ASSETS> 872637
<TOTAL-ASSETS> 914565540
<PAYABLE-FOR-SECURITIES> 33313471
<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 1568210
<TOTAL-LIABILITIES> 34881681
<SENIOR-EQUITY> 0
<PAID-IN-CAPITAL-COMMON> 986069136
<SHARES-COMMON-STOCK> 104411122
<SHARES-COMMON-PRIOR> 97254938
<ACCUMULATED-NII-CURRENT> 337176
<OVERDISTRIBUTION-NII> 0
<ACCUMULATED-NET-GAINS> 0
<OVERDISTRIBUTION-GAINS> (123412312)
<ACCUM-APPREC-OR-DEPREC> 16689859
<NET-ASSETS> 879683859
<DIVIDEND-INCOME> 0
<INTEREST-INCOME> 33902684
<OTHER-INCOME> 0
<EXPENSES-NET> (5298405)
<NET-INVESTMENT-INCOME> 28604279
<REALIZED-GAINS-CURRENT> 1525523
<APPREC-INCREASE-CURRENT> 2668295
<NET-CHANGE-FROM-OPS> 32798097
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> (20496403)
<DISTRIBUTIONS-OF-GAINS> 0
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 13992276
<NUMBER-OF-SHARES-REDEEMED> (8265511)
<SHARES-REINVESTED> 1429419
<NET-CHANGE-IN-ASSETS> (60008201)
<ACCUMULATED-NII-PRIOR> (164064)
<ACCUMULATED-GAINS-PRIOR> 0
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> (124937835)
<GROSS-ADVISORY-FEES> 2048049
<INTEREST-EXPENSE> 0
<GROSS-EXPENSE> 5432785
<AVERAGE-NET-ASSETS> 903024717
<PER-SHARE-NAV-BEGIN> 6.72
<PER-SHARE-NII> .22
<PER-SHARE-GAIN-APPREC> .03
<PER-SHARE-DIVIDEND> (.22)
<PER-SHARE-DISTRIBUTIONS> 0
<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 6.75
<EXPENSE-RATIO> .98
<AVG-DEBT-OUTSTANDING> 0
<AVG-DEBT-PER-SHARE> 0
</TABLE>
<TABLE> <S> <C>
<ARTICLE> 6
<CIK> 0000783740
<NAME> MFS SERIES TRUST X
<SERIES>
<NUMBER> 012
<NAME> MFS GOVERNMENT MORTGAGE FUND CLASS B
<MULTIPLIER> 1
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> JUL-31-1998
<PERIOD-START> AUG-01-1997
<PERIOD-END> JAN-01-1998
<INVESTMENTS-AT-COST> 856548560
<INVESTMENTS-AT-VALUE> 873233419
<RECEIVABLES> 40450382
<ASSETS-OTHER> 9102
<OTHER-ITEMS-ASSETS> 872637
<TOTAL-ASSETS> 914565540
<PAYABLE-FOR-SECURITIES> 33313471
<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 1568210
<TOTAL-LIABILITIES> 34881681
<SENIOR-EQUITY> 0
<PAID-IN-CAPITAL-COMMON> 986069136
<SHARES-COMMON-STOCK> 25837606
<SHARES-COMMON-PRIOR> 42646140
<ACCUMULATED-NII-CURRENT> 337176
<OVERDISTRIBUTION-NII> 0
<ACCUMULATED-NET-GAINS> 0
<OVERDISTRIBUTION-GAINS> (123412312)
<ACCUM-APPREC-OR-DEPREC> 16689859
<NET-ASSETS> 879683859
<DIVIDEND-INCOME> 0
<INTEREST-INCOME> 33902684
<OTHER-INCOME> 0
<EXPENSES-NET> (5298405)
<NET-INVESTMENT-INCOME> 28604279
<REALIZED-GAINS-CURRENT> 1525523
<APPREC-INCREASE-CURRENT> 2668295
<NET-CHANGE-FROM-OPS> 32798097
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> (7603215)
<DISTRIBUTIONS-OF-GAINS> 0
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 701495
<NUMBER-OF-SHARES-REDEEMED> (18115634)
<SHARES-REINVESTED> 605605
<NET-CHANGE-IN-ASSETS> (60008201)
<ACCUMULATED-NII-PRIOR> (164064)
<ACCUMULATED-GAINS-PRIOR> 0
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> (124937835)
<GROSS-ADVISORY-FEES> 2048049
<INTEREST-EXPENSE> 0
<GROSS-EXPENSE> 5432785
<AVERAGE-NET-ASSETS> 903024717
<PER-SHARE-NAV-BEGIN> 6.72
<PER-SHARE-NII> .20
<PER-SHARE-GAIN-APPREC> .03
<PER-SHARE-DIVIDEND> (.19)
<PER-SHARE-DISTRIBUTIONS> 0
<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 6.76
<EXPENSE-RATIO> 1.73
<AVG-DEBT-OUTSTANDING> 0
<AVG-DEBT-PER-SHARE> 0
</TABLE>
<TABLE> <S> <C>
<ARTICLE> 6
<CIK> 0000783740
<NAME> MFS SERIES TRUST X
<SERIES>
<NUMBER> 013
<NAME> MFS GOVERNMENT MORTGAGE FUND CLASS I
<MULTIPLIER> 1
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> JUL-31-1998
<PERIOD-START> AUG-01-1997
<PERIOD-END> JAN-01-1998
<INVESTMENTS-AT-COST> 856548560
<INVESTMENTS-AT-VALUE> 873233419
<RECEIVABLES> 40450382
<ASSETS-OTHER> 9102
<OTHER-ITEMS-ASSETS> 872637
<TOTAL-ASSETS> 914565540
<PAYABLE-FOR-SECURITIES> 33313471
<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 1568210
<TOTAL-LIABILITIES> 34881681
<SENIOR-EQUITY> 0
<PAID-IN-CAPITAL-COMMON> 986069136
<SHARES-COMMON-STOCK> 9789
<SHARES-COMMON-PRIOR> 16882
<ACCUMULATED-NII-CURRENT> 337176
<OVERDISTRIBUTION-NII> 0
<ACCUMULATED-NET-GAINS> 0
<OVERDISTRIBUTION-GAINS> (123412312)
<ACCUM-APPREC-OR-DEPREC> 16689859
<NET-ASSETS> 879683859
<DIVIDEND-INCOME> 0
<INTEREST-INCOME> 33902684
<OTHER-INCOME> 0
<EXPENSES-NET> (5298405)
<NET-INVESTMENT-INCOME> 28604279
<REALIZED-GAINS-CURRENT> 1525523
<APPREC-INCREASE-CURRENT> 2668295
<NET-CHANGE-FROM-OPS> 32798097
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> (3421)
<DISTRIBUTIONS-OF-GAINS> 0
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 693
<NUMBER-OF-SHARES-REDEEMED> (8268)
<SHARES-REINVESTED> 482
<NET-CHANGE-IN-ASSETS> (60008201)
<ACCUMULATED-NII-PRIOR> (164064)
<ACCUMULATED-GAINS-PRIOR> 0
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> (124937835)
<GROSS-ADVISORY-FEES> 2048049
<INTEREST-EXPENSE> 0
<GROSS-EXPENSE> 5432785
<AVERAGE-NET-ASSETS> 903024717
<PER-SHARE-NAV-BEGIN> 6.72
<PER-SHARE-NII> .22
<PER-SHARE-GAIN-APPREC> .03
<PER-SHARE-DIVIDEND> (.23)
<PER-SHARE-DISTRIBUTIONS> 0
<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 6.74
<EXPENSE-RATIO> .73
<AVG-DEBT-OUTSTANDING> 0
<AVG-DEBT-PER-SHARE> 0
</TABLE>
