This study compares self-instructional mastery and nonmastery treatments of a specially developed geography unit to determine if there were differences in learning, retention, and time-to-testing of high, middle, and low aptitude students. Mastery learning is an alternative which gives lower performing students the necessary additional time to learn while the progress of the higher aptitude student is retarded by withholding of additional learning tasks. Twenty grade 7 classes from Savannah-Chatham County School District, Georgia, were chosen as the experimental population. Mastery and nonmastery units were randomly assigned to classes in each school. Tests were administered to measure the students' learning and retention of the content materials. The results indicated that high and middle aptitude self-instructional mastery treatment students retained more than high and middle aptitude nonmastery students. Middle aptitude mastery students learned more than middle aptitude nonmastery students. There was no difference between learning and retention for low aptitude students across treatments. Therefore, the differences between aptitude levels were increased rather than diminished when self-instructional mastery units were used. The educational implications were that mastery treatment facilitated superior achievement at the price of less work covered in the time period, and that mastery treatment for low aptitude students could be effective only with close and careful teacher feedback instead of self-instruction. (DE)