The behaviour of whooper swans Cygnus cygnus at Rostellan Lake, Cork, was studied over the winter of 1991-2. Behavioral patterns were assessed by instantaneous scan sampling and focal sampling. No significant variation was found between the activity patterns of adult and first-winter swans, and thus the flock was treated as a single unit for subsequent analysis. Feeding and comfort activities accounted for 40% and 44% respectively of the total seasonal activity. The other major activities were vigilance and swimming. The main feeding strategies were upending and head and neck submerged whilst feeding on aquatic vegetation. Grazing on agricultural land was observed only once, and only in February. The diurnal feeding pattern varied over the study: sometimes there was a linear increase in the percentage of birds engaged in feeding as the day progressed, while on other occasions a pattern of bimodal peaks was observed.