Seasonal changes in the distribution and composition of common seal haul–out groups were followed in a study area in Orkney, Scotland. A marking programme was also undertaken, using both conventional and radio–tags, to study individual movements between sites and seasonal changes in site–use. Certain haul–out sites were used only in the breeding season, while others were used during the winter. Seals were seen at one site all year round and at another during only the pre–pupping and moult period. On one island where two sites were used during the summer, there were significant differences in the sex ratio of groups at the two sites: at one site males predominated and few pups were seen; on another, nearby, mothers and pups were regularly seen, although the site was also used by males. There was also evidence for segregation of the sexes outside the breeding season. Repeated observations of marked seals showed that seals used several different haul–out sites throughout the year, and that the seasonal changes in abundance at different sites resulted from individual changes in site–use. These changes in site–use are discussed in relation to feeding movements, breeding requirements and the physical characteristics of different sites.