Developmental trajectories of parents’ knowledge of their adolescents’ whereabouts and activities were tested as moderators of transactional associations between friends’ antisociality and adolescent delinquent behavior. 504 adolescents (50% female) provided annual reports (from ages 12 to 16) of their parents’ knowledge and (from ages 13 to 16) their own delinquent behavior and their friends’ antisociality. Parents also reported the adolescents’ delinquent behavior. Growth mixture modeling was used to identify two sub-groups based on their monitoring knowledge growth trajectories. Adolescents in the sub-group characterized by decreasing levels of parents’ knowledge reported more delinquent behavior and more friend antisociality in early adolescence, and reported greater increases in delinquent behavior and friend antisociality from early to middle adolescence compared to adolescents in the sub-group characterized by increasing levels of parents’ knowledge. Transactional associations consistent with social influence and social selection processes also were suppressed in the increasing knowledge sub-group as compared to the decreasing knowledge sub-group.