Kin discrimination should be a tactic in the fitness strategy of individuals. Two postulates derived from inclusive fitness theory that fitness benefits toward kin are correlated with degree of relatedness and that social behavior of members of a kin group is always cooperative are refuted by analyses of the social dynamics of yellowbellied marmots. Cooperative behavior occurs predominantly between mother/ daughter and sister/sister pairs, but such pairs also compete. More distant kin are treated similarly to unrelated animals, social behavior is primarily agonistic. Patterns of space-use, matrilineal formation and organization, social dynamics, and reproductive suppression of close kin suggest that marmots attempt to maximize direct fitness.