In a naturalistic study of age segregation during adolescence, 403 adolescents making up 153 separate groups were observed in three high schools and seven out-of-school settings. Age segregation was extensive, especially in school, among older adolescents in same-sex groups of friends. More mixed-age association was observed out of school among younger adolescents who were in mixed-sex groups containing relatives. Adolescents were most often observed with friends who were other adolescents, next with adults, and least frequently observed with children. Parents, particularly fathers, were rarely present. It was suggested that age segregation during adolescence is the result of a lack of contact with relatives.