This study investigated the effects of grouping patterns in a cooperative learning environment on the mathematics achievement of two high school geometry classes. Heterogeneous, homogenous, random, mixed gender and single gender grouping patterns were coordinated with six similar geometry topics and students were grouped accordingly for a period of 10 weeks. The t test was applied to pretests and posttests after each grouping pattern and only two grouping patterns showed significant results. However, the means, standard deviations, teacher observations, and student responses revealed other implications in relation to grouping and a favorable response to cooperative small group work. (Author)