This paper describes the simultaneous effects, including interactions, of two controlled instructional management variables and two student aptitudes on student achievement using a class-means model. Management variables were time allocated for learning with two levels of 45 and 50 class periods, and source of direction with three levels of student-directed, group-directed, and teacher-directed instructional management. Sample consisted of 1108 students from 75 classes taught by 26 teachers. All instruction used an objectives-based modular system developed for teaching high school life science, Individualized Science Instructional System (ISIS), which has been used nationally since 1971. Student aptitudes considered were reading ability and study orientation. Aptitude-treatment and aptitude-aptitude interactions of practical significance were found. (Author/DS)