Antisocial behavior typically increases during early adolescence, but the possibility of seasonal variation has not been examined. In this study, trajectories of antisocial behavior were estimated for early adolescent boys and girls. Data were obtained from a 3‐year longitudinal study of 11 middle schools in the western United States (n = 5,742), with assessments completed four times per academic year. Antisocial behavior increased steadily throughout 6th grade, but beginning in 7th grade for boys and 8th grade for girls it declined during the school year. Significant increases between Grades 6–7 and 7–8 were found for both genders. Trajectories varied by contextual and individual‐level social victimization and gender. Implications for theoretical development and future studies are discussed.