The role of children's on-task behavior in the prevention of aggressive behavior was assessed among 570 Dutch speaking children followed from second- to third-grade elementary school in Flanders, Belgium. A first objective was to investigate whether individual level variation of on-task behavior moderated the impact of a universal preventive intervention, the Good Behavior Game (GBG), on aggression development, controlling for classroom levels of on-task behavior. The second goal was to study whether improved on-task behavior or reductions in peer rejection mediated intervention impact on children's aggression. Second-grade classrooms were randomly assigned to the GBG or a control condition. Results showed that intervention impact was found only among children who had low-level on-task behavior at baseline. These children showed a decrease in aggression when in the GBG condition, which was not found among control group children. The reduction in aggression among low on-task children was mediated by reductions in peer rejection. No mediation effect of on-task behavior was found. These results suggest that the effect of a universal preventive intervention may depend upon initial levels of on-task behavior and that improvements in social relations with peers may explain the reductions in aggression among these low-on task children. (Contains 2 tables and 4 figures.)