Measures of delinquent peers have been found to predict a wide variety of antisocial behaviors, but few studies have examined the role that biological factors may play in moderating the delinquent peers–delinquency relationship. Using a sample of adolescent males drawn from the National Longitudinal Survey of Adolescent Health (N = 3,557), we explore whether variants of the dopamine transporter gene (DAT1) interact with associations with substance-using peers to affect self-reported delinquency. Results of negative binomial regressions reveal that affiliations with delinquent peers interact with the 10R allele of DAT1 to influence offending, net of control variables for self-control, and respondent’s substance use. Most important, a statistically significant effect of affiliations with delinquent peers on delinquency is only evident for males who carry two 10R DAT1 alleles (vs. zero or one). Thus, DAT1 may be implicated in antisocial behavior by rendering some individuals more susceptible to the influences of delinquent peers. Implications for criminological theory are discussed.