Colleges typically group students homogeneously in classes by means of both admission requirements and course prerequisites, but when professors form cooperative learning groups within classes they generally use heterogeneous grouping. Authors compared heterogeneously and homogeneously grouped cooperative learning groups in six paired classes, taught by the same professor using matching syllabi and assignments. Overall, homogeneously grouped students (who were grouped based on achievement on the first test given in the course) significantly outperformed heterogeneously grouped students on the final examination. High- or average-achieving students particularly benefited from homogeneous grouping. Low achievers did equally well in either type of group. (Contains 4 tables.)