AT THE FLORIDA STATE UNIVERSITY SCHOOL, A LONGITUDINAL STUDY ATTEMPTED TO FIND OUT IF KINDERGARTEN THROUGH THIRD GRADE CHILDREN WHO ATTENDED THE EXTENDED SCHOOL YEAR OF THE TRIMESTER SYSTEM SHOWED SIGNIFICANT DIFFERENCES IN ACHIEVEMENT AND ADJUSTMENT AS COMPARED WITH CHILDREN OF A SIMILAR MEAN IQ WHO ATTENDED ONLY DURING THE REGULAR SCHOOL YEAR. THE URBAN AREA SUBJECTS HAD THESE ATTENDANCE PATTERNS, (1) 38 CHILDREN ATTENDED FOR THREE SUMMERS, (2) 38 FOR NO SUMMERS, (3) 44 FOR ONE SUMMER, AND (4) 43 FOR TWO SUMMERS. FIRST, SECOND, AND THIRD GRADE ACHIEVEMENT WAS DETERMINED BY THE METROPOLITAN ACHIEVEMENT TEST AND THE DEVELOPMENTAL READING TEST. ADJUSTMENT WAS EVALUATED BY THE HAGGERTY-OLSON-WICKMAN BEHAVIOR RATING SCHEDULES, WHICH USED TEACHERS' RATINGS, AND THE CALIFORNIA TEST OF PERSONALITY. ALL TESTS WERE GIVEN EACH OCTOBER AND MAY FOR A THREE-YEAR PERIOD. THE SCIENCE RESEARCH ASSOCIATES MENTAL ABILITIES TEST WAS GIVEN TO ALL SUBJECTS BEFORE MID-OCTOBER OF THE FIRST YEAR. ANALYSIS OF VARIANCE OF THE DATA PERMITTED RESULTS TO BE ADJUSTED IN TERMS OF VARIATION DUE TO INTELLIGENCE. ALTHOUGH THE EVIDENCE IS INCONCLUSIVE, FINDINGS INDICATE THAT EXTENDING THE LENGTH OF THE SCHOOL YEAR HAS A NEGATIVE EFFECT ON CHILDREN'S ACHIEVEMENT AND ADJUSTMENT. FURTHER TREND ANALYSES OF THE DATA COLLECTED WILL DETERMINE THE MOST CRITICAL AGE LEVELS IN RELATION TO SCHOOL YEAR LENGTH. A STUDY WILL ALSO BE MADE TO FIND OUT WHY CHILDREN DO OR DO NOT ATTEND SUMMER SCHOOL. AN EXTENSIVE BIBLIOGRAPHY IS INCLUDED IN THIS REPORT. (MS)