The simultaneous effects of the two instructional variables of degree of teacher direction (student-directed versus teacher-directed) and time allowed for study and the student variable of general ability are described for student achievement in three life-science instructional modules covering approximately three months. The instructional variables were controlled in this study; that is, high school teachers were randomly assigned to one of six combinations of the student-directed versus teacher-directed method variable and the allowed time variable with three levels. Between-class, within-class, and between-student analyses of data for 912 students in 43 classes resulted in a description of effects complicated by the presence of interactions among the three independent variables. For example, there was a disordinal interaction between instructional method and allowed time, so that statements about the relative superiority of the student-directed and teacher-directed approaches must be qualified by specification of allowed time. In any event, the estimated differences were modest, suggesting that the factor of possible achievement differences should not be dominant among the many factors considered in selecting an approach. A very modest ordinal aptitude (ability)-treatment-interaction (ATI) was also found, although the main effect of ability was quite strong. The allowed time effect suggested the possibility of decreased achievement under some circumstances when allowed time is increased, due perhaps to increased misuse of time and associated disruption.