Examined was whether within-class social comparison processes mediate the effects of ability grouping in mathematics on self-concept. The sample was 149 students in seventh-grade classrooms in two schools, one practicing within-classroom grouping and the other using between-classroom grouping. It was predicted that, among students who frequently use social comparison information to evaluate their ability, self-concept of mathematics ability ought to be more strongly related to the level of one's group if one is in a school that practices within-classroom grouping by ability. Among students who do not use social comparison information often, self-concept of mathematics ability should be equally related to the level of one's group, whether within-or between-classroom grouping practices are used. These predictions were partially confirmed, although complicated by interaction effects involving gender. The importance of these findings for the literature on social comparison of ability and on ability grouping in schools is discussed. (Author/MNS)