In this study, I examined the relationships between ethnic identity, social support within one’s own group, activity with one’s own group, and readiness to cooperate with people of another group. The hypothesis that there will be a stronger link between ethnic identity and prosocial activity in a minority group was tested using a questionnaire collected from 60 Byelorussians and 62 Poles living in the same villages of eastern Poland. The analyses showed that Byelorussians had lower ethnic identity and social support than Poles but higher readiness to cooperate with Poles than the reverse. For the Byelorussians, the correlational analysis revealed significant coefficients between ethnic identity and social support, between ethnic identity, social support, and prosocial activity, and between social support and readiness to cooperate, whereas the relationships among these variables were not significant in the Polish group. On a cognitive level, therefore, the members of the dominant group identified more with the nation and country than the members of a minority, whose ethnic identity was more salient in social attitudes and ingroup and outgroup activities.