A cross-societal experiment with 49 Australian and 56 Japanese participants examined if the group heuristic account of ingroup-favoring behavior in a Prisoner's Dilemma game can be extended beyond the minimal group situation to a situation involving an enduring social category (i.e. participant's nationality). Participants played a Prisoner's Dilemma game five times, each time with a different partner. Two of the five partners were ingroup members, two were outgroup members, and the nationality of one partner was not known. Furthermore, one of the two ingroup (or outgroup) partners knew that the participant was a member of the same (or the other) nationality, and the other did not know it. The results indicated that the knowledge that the partner had about the nationality of the participant exerted an effect only when the partner was an ingroup member. No major difference was found between Australian and Japanese participants. An outgroup-favoring cooperation pattern was observed, but that pattern was shown to be a result of fairness concerns among Australian participants and of positive stereotypes of Australians among Japanese participants.