A gap exists between studies of gender and language and studies of bilingualism. The considerable attention given by studies of discourse style to gender differences in friendship patterns is not reflected in research on bilingual communities. This ethnographic case study of high school students in the Barcelona area shows that gender differences in peer group structure can affect the use of the bilingual repertoire, even when there are no apparent sex differences in second language acquisition. In this setting, girls' friendship circles are more solidary and cohesive than boys'. Moreover, girls' groups are ethnically and linguistically homogeneous, while boys' social circles can be ethnically mixed and internally differentiated linguistically. Girls' friendships set stronger constraints on language behavior. In turn, boys and girls reap different social benefits and costs for their linguistic choices, with more serious consequences for girls' social identities and acceptance by peers. (Gender, sex differences, bilingualism, peer groups, language acquisition, social networks, language shift)