We employed a five-month longitudinal study to test a model in which the association between anxiety and depression symptoms is mediated by peer relations difficulties among a sample of 91 adolescents ages 14-17 (M=15.5, SD=.61) years. Adolescents completed measures of anxiety symptoms, depression symptoms, peer group experiences (i.e., peer acceptance and victimization from peers), and friendship quality (i.e., positive qualities and conflict). As hypothesized, Time 1 anxiety symptoms predicted Time 2 (T2) depression symptoms, and this association was mediated by T2 low perceived peer acceptance and T2 victimization from peers, both of which emerged as unique mediators when they were considered simultaneously in the model. Contrary to expectations, qualities of adolescents' best friendships at T2 did not emerge as mediators and were largely unrelated to symptoms of anxiety and depression. Implications of the findings include the importance of addressing peer relations difficulties, especially peer acceptance and victimization, in the treatment of anxiety and the prevention of depression among anxious youth.