The social behavior of 208 mentally retarded residents in 18 group homes was observed and analyzed in terms of their (a) informal grouping behavior, (b) affiliation, and (c) intense social relationships ("friendships"). Moderately to mildly retarded residents engaged in "social" types of behavior in groups (primarily dyads) and "neutral" types of behavior when alone. In contrast, profoundly retarded residents did not behave differently when others were present, although they did display social types of behavior. Residents' affiliation behavior was not as related to personal characteristics (sex and intelligence) as it was to (a) size of the group home, (b) average intelligence of the residents in the home, (c) sex ratio in the home, and (c) homogeneity of the residents' backgrounds. Although residents in large homes affiliated more extensively with others, intense friendships were identified and described. In total, the results support the view that the group home environment can influence residents' affiliation and friendship.