Designated time allotments for the various course offerings of the elementary schools have evolved with little research showing that present time allotments are the best possible. The problem of this study was to determine the relationship between varying lengths of class periods and pupil achievement in reading, arithmetic, and language. Mental maturity and achievement tests were administered to 713 sixth grade pupils who attended each of the selected schools for all their intermediate elementary grade education. The study shows that maximum class period lengths resulted in greater pupil achievement in every area tested for those with intelligence quotients of 115 or more. For the average student, longer class periods resulted in significantly higher achievement in the areas of arithmetic and language. [Table V may reproduce poorly in hard copy due to small print.] (MF)