The purposes of the study were to (a) determine whether there was a significant difference in the mathematics achievement of students which may be attributed to any of the nine variations in trimester scheduling during the two-year period, and (b) to analyze the implications of trimester scheduling as they apply to teachers, counselors, and administrators in the middle school program. On the basis of careful treatment and analysis of data collected to ascertain the effect of time-lapse scheduling on mathematics achievement in arithmetic computation, arithmetic concepts, and arithmetic applications, the following conclusions, limited to the population studied, were drawn: 1. None of the nine time-lapse schedules had a significant differential effect on mathematics achievement. 2. The results of this study may be generalized to include the high, middle, and low achievement segments of the target population of students in all middle schools in the school district of this study. 3. Middle school administrators and counselors may schedule students into any convenient sequence of courses without fear of relative detrimental consequences to mathematics achievement. 4. Teachers need not be concerned that "learning loss" in mathematics due to different time-lapse schedules of the students will require different amounts of review prior to beginning work in a new trimester. 5. The Intensified Learning Plan is a viable program in mathematics in the school district's middle school plan of organization and scheduling.