Existing models of kin selection implicitly assume that the intensity of density-dependent mortality depends on the size or density of the entire deme. If individuals compete for locally limited resources, then mortality may also depend on local population density. In this paper, I derive conditions for evolutionary equilibrium when the fitness of an individual depends on (1) the phenotypes of a local group of conspeeifics with which it interacts and (2) the size of the local group. In general, altruistic behaviours which reduce the individual fitness ofthe actor but increase that ofthe recipient are favoured towards individuals whose coeliicient of relationship exceeds a threshold value. ‘Harming’ behaviours which reduce the individual fitness of both the actor and the recipient are favoured towards individuals below this threshold value. T he magnitude of this threshold depends on the nature of the processes that cause density-dependent mortality and the average coemcient of relationship in the group.