Previous research has indicated that two of the strongest correlates of delinquent behavior are gender and association with delinquent peers. While a few studies have examined how gender affects the relationship between association with delinquent peers and delinquent behavior, the current study extends this research by analyzing longitudinal data on 1,600 adolescents to examine the extent to which internal and external constraints influence the relationship between gender, delinquent peers, and delinquent behavior. Participants were 1,600 10th-grade students at 9 high schools in South Carolina who completed the questionnaire survey during their English course in 1981 and then again during their junior year in 1983. Questionnaires measured self-reported delinquency, moral beliefs, association with delinquent peers, perceived sanction certainty for delinquent behavior, and perceived sanction severity. Results of regression analyses indicated that the relationship between delinquent peer association and delinquent behavior varied by gender, with delinquent peer association being a better predictor of delinquent behavior for boys than for girls. These results remained significant after controlling for prior delinquency. The effect of delinquent peers on delinquent behavior also varied according to the level of internal and external constraints. Future research should focus how gender definitions in society may differentially influence perceptions of internal and external constraints for adolescents. Tables, notes, references