This study was designed to investigate a genetic moderation effect of dopamine receptor 4 gene (DRD4) alleles that have seven or more repeats (long alleles) on an intervention to deter drug use among rural African American adolescents in high-risk families. Adolescents (N = 291, M age = 17) were assigned randomly to the Adults in the Making (AIM) program or to a control condition and were followed for 27.5 months. Adolescents provided data on drug use and vulnerability cognitions three times after pretest. Pretest assessments of caregiver depressive symptoms, disruption in the home, and support toward the adolescent were used to construct a family risk index. Adolescents living in high-risk families who carried at least one DRD4 long allele and were assigned to the control condition evinced greater escalations in drug use than did (a) adolescents who lived in high-risk families, carried the DRD4 long allele, and were assigned to AIM, or (b) adolescents assigned to either condition who carried no DRD4 long alleles. AIM-induced reductions in vulnerability cognitions were responsible for the Family Risk × AIM × DRD4 status drug use prevention effects. These findings support differential susceptibility predictions and imply that prevention effects on genetically susceptible individuals may be underestimated.