Prompted by research suggesting females' self-concepts are more interpersonally rooted than males', I compare girls' identity changes in reaction to relationships in new social contexts with boys', testing whether identity change processes are the same for each sex. I use survey responses from 320 summer program students about five activity areas: (1) science & technology; (2) computers; (3) athletics & recreation; (4) beliefs & interests; and (5) arts & literature. While girls become more attached to and involved with others, their identity processes are equivalent to those of boys. Girls change more, but their change is rooted in greater sociability, not higher reactivity to new relationships. Findings vary by relationship and activity types, indicating sex differences may reflect gender role expectations.