Designers of computer-based practice systems give primary attention to aspects of cognitive learning. However, observations of students practicing arithmetic, with two CAI systems widely used in Israel, revealed factors in the computer learning environment that substantially affected students' social behavior, particularly with respect to competition and cooperation in the course of learning. A questionnaire was designed on the basis of the observations to examine the effects of these factors in CAI work. It was administered to 457 fourth graders in 16 classes (two CAI systems x two SES categories x two schools x two classes). While neither CAI system was intended to foster competition, the observations and questionnaire data suggested that one encourages social comparison and competition whereas the other discourages these features and encourages cooperation and peer help. Boys appear to be significantly more competitive than girls. One system, compared with the other, significantly increases competitive behavior of high achievers (but not of low achievers); results in teachers' greater encouragement of social comparison and competition among students; and, as used in the schools we studied, causes disadvantaged students rather than advantaged students to develop negative feelings because of low CAI performance. The other system discourages competitive behavior and encourages cooperation. All these findings bear important implications for the instructional design of computer-based learning environments.