Many animal species segregate by sex. Such segregation may be social in nature, or ecological, or both. Grey seals (Halichoerus grypus), like many large mammals, are sexually size dimorphic. In size dimorphic species, allometric differences in morphology, metabolic rate and reproductive costs are likely. Such differences may require the sexes to use different foraging strategies or different habitats. To investigate sexual segregation of habitat in grey seals, we used satellite tracks from 95 (male 46; female 49) adults breeding at Sable Island, Nova Scotia (44 degrees N, 60 degrees W) collected from 1995 to 2005. Location estimates were made from satellite fixes using a state-space movement model to estimate true locations and regularize them in time. Location estimates were used to calculate home range kernels of male and female habitat use each month. Month by sex kernel home ranges revealed striking differences and dynamics in habitat use between males and females on spatial scales broader than most terrestrial examples and at temporal and spatial resolutions rarely available for marine species. Differences were most pronounced just before (October-December) and immediately after breeding (February-March). During both periods, males primarily used areas along the continental shelf break, while females mainly used mid-shelf regions. Coupled with previously identified sex-specific seasonal patterns of energy storage, diving and diet, our findings suggest that males and females differ profoundly in their spatial foraging strategies. These differences may serve to maximize fitness by reducing intersexual competition during key foraging periods.