Purpose: Low heart rate (HR) has been linked to antisocial behavior (ASB). However, the effect of low HR may be mediated by affiliation with bullies. We hypothesized that individuals with low HR are more likely to affiliate with bullies and in turn are influenced by these peers. Methods: Data come from two waves of a subsample of the TRAILS study (N = 809; 44.0% boys; mean age of 11.0 years at T1 and 13.5 years at T2). ASB was measured using the Antisocial Behavior Questionnaire at both waves. HR was assessed during rest at T1. Affiliation with bullies was assessed via peer nominations at T1. Possible gender differences were taken into account, and all analyses were adjusted for family context (i.e., family breakup and socioeconomic status). Results: Regression analyses showed that lower HR was only associated with ASB in (pre)adolescents who affiliated with bullies. Moreover, the effect of lower HR on boys' ASB was partly mediated by affiliation with bullies. Conclusions: Our findings show that (pre) adolescents, and in particular boys, seem to be in environments that match their biological disposition and in turn are shaped by this environment. (C) 2013 Published by Elsevier Inc. on behalf of Society for Adolescent Health and Medicine.