We tested the effects of instructional method (lecture vs. mastery) and aptitude (high, medium, low GPA levels) on students' academic performance and achievement motivation goals. Students enrolled in a developmental psychology course received either traditional lecture or mastery (Keller's Personalized System of Instruction [PSI]) instruction. PSI students ended the semester with higher final examination scores than lecture students; the difference was greatest for the lowest GPA group. Low-aptitude PSI students showed increases in learning goals over time when compared to low-aptitude lecture students. The academic performance and learning goals of PSI students with the lowest grade point averages (GPAs) approached those of lecture students with the highest GPAs.