The effect on achievement of the timing of the transition from elementary to intermediate school was investigated. Focus was on determining whether it matters if children transfer to an intermediate school at grade 6 or at grade 7. The Otis-Lennon School Ability Test was administered to grade 5 students in spring of 1981. The sixth and seventh-grade achievement scores on the Stanford Achievement Test of 1,140 of these students who entered middle school in the sixth grade were compared to those of 1,158 students who entered junior high school in the seventh grade. Overall, sixth-grade students in self-contained classrooms with teachers instructing in all subjects performed as well or better than did students in middle schools with separate teachers for mathematics, science, social studies, and language arts. Results suggest that student achievement is affected by the transition into middle school, whether it occurs at grade 6 or grade 7. The advantage that the elementary school sixth-grader held over the middle school sixth-grader was lost by the end of grade 7 when both groups were attending intermediate schools. Seven tables contain study data. (SLD)