Consistent with a Diathesis × Stress model, it was hypothesized that anxious solitude (individual vulnerability) and the middle school transition (environmental stress) would jointly predict peer exclusion and victimization trajectories. Youth (N = 688) were followed from 3rd through 7th grade, with the middle school transition in 6th grade. Peer-reported peer exclusion and physical victimization trajectories across the middle school transition were modeled with piecewise growth curves. As expected, anxious solitude predicted elevated exclusion and victimization in both elementary and middle school. Nonetheless, exclusion and victimization declined after the transition on average, and anxious solitary youth versus average youth experienced greater relative declines. The pattern of results suggests that the collective renegotiation of peer relations after the transition, rather than posttransition decline in classroom emotional support, contributed to the posttransition decline in peer mistreatment.