Although physical activity participation has numerous physiological and psychological benefits, inactivity rates remain high, and a greater understanding of the factors that drive participation is needed. Growing evidence indicates that (1) the strength of individuals' social identification as a member of a particular physical activity group (e.g., an exercise group or sports team) is positively associated with their group-relevant participation, and (2) physical activity leaders (e.g., exercise group leaders, coaches, and captains) can foster members' identification, and thus their greater group-relevant participation. Extending previous crosssectional research, we examined relationships over time between sports group members' perceptions of their leaders' engagement in identity leadership, their group identification, and attendance. Participants (N = 186) from amateur sports teams completed measures of identity leadership, group identification, and attendance on two occasions, eight weeks apart. Lagged regressions indicated that perceptions of leaders' engagement in identity leadership at Time 1 predicted members' group identification at Time 2, controlling for their group identification at Time 1; and members' group identification at Time 2 was associated with their attendance at Time 2, controlling for their attendance at Time 1. Mediation analysis demonstrated a significant indirect effect of perceptions of leaders' engagement in identity leadership on group members' attendance through greater group identification. Findings provide evidence of the participation-related benefits of forming, and maintaining, strong social identities in physical activity settings, and point to the role leaders can play in fostering members' sustained identification and participation.