We report a study aimed at understanding the effects of classroom normative influences on individual aggressive behavior, using samples of 614 and 427 urban elementary school children. Participants were assessed with measures of aggressive behavior and normative beliefs about aggression. We tested hypotheses related to the effects of personal normative beliefs, descriptive classroom norms (the central tendency of classmates' aggressive behavior), injunctive classroom normative beliefs (classmates' beliefs about the acceptability of aggression), and norm salience (student and teacher sanctions against aggression) on longitudinal changes in aggressive behavior and beliefs. injunctive norms affected individual normative beliefs and aggression, but descriptive norms had no effect on either. In classrooms where students and teachers made norms against aggression salient, aggressive behavior diminished over time. Implications for classroom behavior management and further research are discussed.