This study is an assessment of the effect of three criterion mastery levels and aptitude on the achievement, retention, and attitude of seventh grade students using a population geography text. The three criterion levels used were 90 percent, 80 percent, and 70 percent of the correct responses on each of 41 lessons in "Population Growth in the United States and Mexico." Aptitude was measured by a word meaning text. Aptitude was a major independent variable because of the premise that mastery procedures may overcome achievement difficulties of low aptitude students. Contrary to most of the literature on mastery learning, the results indicate that mastery on formative exercises did not contribute to higher levels of achievement on the summative test. The study also failed to show that achievement, retention, or attitude toward the unit differed among the three criterion groups. The study confirmed that previous knowledge, as measured by a word meaning test, was a more potent factor in achievement than were differential criterion levels. (Author/DE)