We studied the impact of MAOA genotype, childhood sexual assault, and harsh discipline on clinical externalizing symptoms (substance problems, adult antisocial behavior, and conduct disorder). Participants were 841 individual twins from the Minnesota Twin Family Study assessed through age 25. MAOA genotype was not associated with differences in any phenotype, nor was there a significant interaction between MAOA and harsh discipline for any phenotype or a significant interaction between MAOA and childhood sexual assault for substance problems. We found evidence that childhood sexual assault interacted with MAOA genotype to predict antisocial behavior and conduct disorder symptoms. Individuals with the low MAOA activity genotype who reported childhood sexual assault had more symptoms than individuals with either the high MAOA activity genotype and/or no history of childhood sexual assault. These findings suggest that the previously reported interaction between MAOA and childhood maltreatment may be specific to the antisocial subset of externalizing disorders.