One hundred and thirty-six 5- and 6-year-olds participated in this study, which investigated the extent to which cooperative, trusting behavior could be demonstrated between Mexican-American, Negro, and Anglo-American children. Also considered were some of the basic variables which were important in the development of such behavior. Similar and dissimilar ethnic-group pairs were placed into immediate or delayed reward groups. Each child was given a choice of either competitive or cooperative behavior in relation to an unseen partner's behavior. Male subjects showed no significant differences in behavior. Female similar ethnic pairs were more cooperative than were dissimilar ethnic pairs, with the exception of Mexican-American and Negro pairs. Anglo-American females competed the most. Type of reinforcement and number of trials did not affect cooperative behavior. Greater maturity and understanding of ethnic mores might have been responsible for female behavioral differences. A bibliography is included. (MS)