Mutual antipathies (when two children or adolescents dislike one another) were studied among 2,348 school–age children and 2,768 adolescents to determine incidence, gender and age differences, and implications for social adjustment. The children were more frequently involved than were the adolescents in same–sex antipathies but not mixed–sex ones. Boys were involved more frequently than were girls in same–sex antipathies, but involvement in mixed–sex antipathies was comparable for the two genders. With peer rejection scores used as a covariate, same–sex antipathies were associated with antisocial behavior and social withdrawal among children and adolescents of both genders and, in addition, to emotionality and lack of friendship support among adolescents. Mixed–sex antipathies were related to social adjustment depending on gender: these antipathies were related to antisocial and bullying behavior in boys but not girls; and to nonaggressiveness, victimization, lesser cooperation, shyness, and depression in girls but not boys. Mutual antipathies thus appear to be concomitants of adaptational risk in both childhood and adolescence.