Animal contests usually concern ownership or access to a single resource. Asymmetries in the resource holding power and the value of the resource to each of the contestants may be assessed by the animals and these assessments used to determine the winner of the encounter ( Parker, 1974). Hermit crabs, however, may exchange resources (shells) at the end of an encounter. This feature has led to the proposal of alternative hypotheses that these encounters are a process of (a) negotiation ( Hazlett, 1978) and (b) aggression ( Elwood & Glass, 1981). In the present study we have examined data on hermit crabs within the framework of theory on asymmetric contests with incomplete information (Hammerstein & Parker, 1983). A slight modification of this model provides a good fit with the data and will still allow its use in more conventional contests. The model of negotiation, however, provides a poor fit with the data.