In this longitudinal, observational study of heterosexual interaction at middle school dances we examined the degree to which boys' and girls' groups became more gender integrated over time. The results show groups became more integrated over time with the pattern differing by gender. Boys had a relatively low level of contact with girls over the early months with an accelerated increase in the latter months. Girls had an increase in contact with boys over the early months but then a decrease in contact over the latter months. Further, changes in the dynamic covariate, aggression, were associated with changes in integration, and the static covariate, initial physical attractiveness, was predictive of integration trajectories. Physical attractiveness and aggression did relate to gender integration, but counter to our hypotheses, each strategy did not vary by gender, as predicted by sexual selection theory. Results are discussed in terms of different methods in assessing the roles of aggression and physical attractiveness in gender integration. (Contains 4 figures and 4 tables.)