The cooperative–competitive behavior of children (Grades 4–6) in Papua New Guinea was assessed by 2 experimental techniques that have previously been used within other countries to demonstrate ethnic differences. The results of Exps I and II, in which the Madsen cooperation board was used, indicate significantly more cooperation between Ss of an intact tribe than between Ss in a heterogeneous urban setting. In Exp III, the marble-pull apparatus was used to assess the cooperative interaction of Ss from 10 sites that varied in tribal intactness, degree of Western contact, and urbanization. Results demonstrate the significance of primary group identification as a determinant of cooperative behavior.