In the present study, processes of interaction between a cooperatively oriented and a non-cooper-atively oriented group were analyzed. Fifty-four female students were divided into nine pairs of three-person groups, and each pair of groups having different orientations participated in 15 trials of the Prisoner's Dilemma Game. After completion of the trials, each subject was asked to evaluate own and opposite group's intentions in the game, and also to evaluate how successful own group was. Results obtained were as follows: (1) In the course of the trials, non-cooperative groups remained constant in their strategies, whereas cooperative groups became less cooperative, assimilating themselves to their opposite groups. (2) Non-cooperative groups perceived their opponents as similar to themselves, whereas cooperative groups perceived their opponents as different from them and had strong feelings of failure. (3) Cooperative groups attributed responsibility for the game outcome more to non-cooperative groups than to themselves.