The effects of interpersonal cooperation, competition, and individualistic efforts were compared on a categorization and retrieval, a spatialreasoning, and a verbal problem-solving task. Forty-five first-grade children were randomly assigned to conditions stratified on the basis of sex and ability, so that an approximately equal percentage of males and females and high, medium, and low ability children were included in each condition. The results indicate that on all three tasks students in the cooperative condition achieved higher than did those in the individualistic condition, and on two of the three tasks students in the cooperative condition achieved higher than did those in the competitive condition. There were no significant differences between the competitive and individualistic condition. Students in the cooperative condition used higher quality strategies on the three tasks than did those in the other two conditions, and they perceived higher levels of peer support and encouragement for learning. High ability students in the cooperative condition generally achieved higher than did the high ability students in the competitive and individualistic conditions. This study has three purposes. The first is to compare the relative effects of cooperative, competitive, and individualistic conditions on problem-solving performance. The second is to examine three possible influences on the problem-solving success of cooperative groups: (1) quality of strategy used, (2) medium and low ability students benefiting from their interaction with high ability group members, and (3) increased incentive to succeed resulting from peer support and encouragement for achievement. The third purpose is to provide further validation for a set of instructional strategies.