This study examined the role of natural peer-group selection and socialization processes in children's school motivation. At the beginning of the school year, self- and teacher reports of classroom engagement were obtained for 109 students in 2 4th- and 2 5th-grade classrooms. On the basis of interviews with 57 students, composite maps of the social networks in these classrooms were constructed, which were found to be reliable across reporters and composed of children with similar motivational orientations. Longitudinal analyses of a 4th-grade classroom across the school year indicated continuity in the motivational composition of peer groups, despite considerable changes in individual memberships. Evidence was found for motivationally based group selection across time and for group socialization of individuals' engagement.