This quasi-experimental study compared children who were (or were not) exposed to a 28-week program designed to prevent aggressive behavior by teaching pro-social skills and social knowledge. Children in the experimental group increased significantly in teacher-ratings of cooperation, assertion, self-control, and total social skills, as well as in social knowledge as measured through an interview procedure. Children in the comparison group showed no significant improvements. Neither group showed changes in teacher ratings of externalizing or internalizing problem behaviors. Preliminary analyses suggest that the experimental group also showed reduction in observed aggressive behavior, while the comparison group did not. Changes within the experimental group differed according to whether the teacher had initially identified a child as high or as low in social competence. High competence children showed significant increase in cooperation and in total social skills, while low competence children showed significant increase in cooperation, assertion, total social skills, and social knowledge. The sample size for this study was small (37 subjects), and should be replicated with a larger sample. Results do, however add quasi-experimental support for the effectiveness of the program, and suggest that changes are due to the program, rather than to maturation.