The purpose of this paper was to explore the age-segregation of U.S. children in two situations--children's spontaneous groups and adult-organized activities--and to examine the developmental changes in children's companionship between infancy and adolescence. The companionship of 400 urban children ranging in age from 1 to 12 years was determined by spot observations, phone contacts, and interviews. Results showed that while the likelihood of being with same-age companions increased with age throughout childhood, children were frequently in the company of children who differed in age from themselves by at least 2 years; that the likelihood of being in the company of children increased until 7-8 years and then decreased; and that the likelihood of being with an adult companion decreased significantly over childhood. Results also indicated that adult-organized summer programs were not more age-restricted than children's own groups. Comparisons of a subset of these data to a rural Guatemalan sample demonstrated very similar patterns of age-segregation, suggesting that U.S. children's social contacts are less restricted than many authors assume. (Author/MP)