Hermit crabs are marine crustaceans that must inhabit empty gastropod shells for shelter and protection from predation, desiccation and osmotic stress. In New England, the hermit crab (Pagurus longicarpus) commonly occupies two different types of gastropod shells: the mud snail (Nassarius obsoleta) and the periwinkle (Littorina littorea). We conducted several laboratory experiments in which we examined the preference of hermit crabs for intact and damaged shells of the mud snail (N. obsoleta) and of the periwinkle (Littorina littorea). Our results suggest that the hermit crab, Pagurus longicarpus, shows a strong avoidance of damaged shells but that known about the relative importance of mud snail shell characteristics in determining hermit crab shell choice. I examined which features of the mud snail shell hermit crabs choosing shells assess. Results of my laboratory shell choice experiments revealed that hermit crabs show a significant preference for intact optimal size shells and that they avoid shells that are too small or damaged when they are given a choice.