This study tested the hypothesis that cooperative learning practices in heterogeneous classes would enhance students' academic motivation and achievement. Half the seventh graders entering a junior high school were randomly assigned to hetergenous classrooms in which their teachers implemented Student Team Learning (STL) techniques (Slavin, 1986). The remaining students were assigned to homogeneous ability-grouped classrooms. The use of STL in heterogeneous classrooms produced achievement benefits, with effects stronger for course grades and end-of-year grades than for results on a nationally-standardized achievement test or a state criterion-referenced test. The effects of STL in heterogeneous classes varied by subject area, with consistent positive effects in language arts, math, and science, and inconsistent effects in mathematics and social studies. The effects of STL on motivation also varied by subject, with STL raising motivational variables in language arts, math, and science, but not in social studies. Such variations can be attributed to variations in teachers' implementation of STL. Ten tables present study findings, and an appendix lists indicators of motivational constructs. (Contains 32 references.) (SLD)