VHF radio-telemetry was used to study individual and seasonal variation in common seal haul-out behaviour in a study area in Orkney, Scotland. The timing of the annual cycle was also described and seasonal changes in behaviour, distribution and abundance were related to the constraints of breeding and the moult. There were consistent seasonal trends in site-use, with some sites being used during the breeding season, and others during the winter. Sex differences in site-use also occurred during the summer, with mothers and pups being seen regularly at some sites, and males predominating at others. A high degree of individual, sex-related and seasonal variation in haul-out behaviour was found. In general, seals spent most time in inshore waters, and hauled out frequently, during the summer. For males, this was especially marked during the moult, whereas females came ashore most regularly during the pupping period. Outside the breeding season, seals left the study area for longer periods, of several days, but regularly returned between trips to haul out. These longer winter trips are believed to represent periods of more intense feeding activity, and it is suggested that the constraints of breeding and the moult restrict seals to shorter feeding trips during the summer. Changes in haul-out behaviour during the moult resulted in common seals being most abundant on land in August. Behaviour was also less variable during the moult, and it is suggested that population surveys should be made at this time of year. An aerial survey was made over Orkney in August 1985 and, in conjunction with telemetric data on haul-out frequency, the results were used to produce a preliminary estimate of the size of the population in this area.