This study examined predictive relations between testosterone levels and conflict tactics in late adolescent heterosexual couples. It was hypothesized that testosterone (T) levels within the couple would predict aggressive conflict tactics, with the link between testosterone and aggression expected to be stronger for males. Results of hierarchical linear modeling analyses showed that the interaction of his and her T levels predicted the male's frequency of both psychological aggression and physical assault within the relationship; when both he and she were concordant for higher or lower levels of T for their gender group, he was more aggressive than if they had complementary levels. There was also a trend toward this same synergistic interaction in predicting females' physical assault. Possible explanations from primate research and evolutionary theory, as well as how these findings fit with previous indications from marital research, are discussed.