In this 3-year prospective study (N = 458) spanning a transition to middle school, bidirectional associations between children's relations with schoolmates (social preference and impact) and behaviors (physical and relational aggression, prosocial behavior) were examined using structural equation modeling. Gender moderation of all estimated paths was examined. There was no gender difference in relational aggression in Grade 3; girls were more relationally aggressive than boys in Grade 6. Males were more physically aggressive than females; females were more prosocial and preferred by peers. Longitudinal and bidirectional associations between relations with peers and behaviors were found. Compared to one-group models of social preference and impact, gender moderation models improved model fit significantly. With regard to social preference and children's behaviors, gender moderated concurrent, but not longitudinal, associations. With regards to social impact and children's behaviors, gender moderated both concurrent and longitudinal associations. There was moderate stability of behaviors, including relational aggression.