Seasonal and diurnal changes in territory size and frequency of aggressive interactions of various intensity were examined in colonies of the black-headed gull (Larus ridibundus) on river islands and ponds of central Poland. The highest frequency of the total aggression and the largest territories were recorded before the egg-laying stage, this being related to the establishing of territories, defence of the mate, and defence of nest material. Later in the season both aggression and territory size declined. Unlike the large species of gulls, black-headed gulls did not increase their aggression and territory size during hatching of the young. High-intensity aggressive behaviours (choking, attacking and fighting) were not so frequently displayed as low-intensity aggressive postures (upright, long call, forward) in each phase of the breeding cycle. Presumably this was because the former were more costly in terms of energy. Peak territorial activity occurred in the late afternoon. An increase in the frequency of intense aggressive behaviours (attack, fight) at that time was combined with an increased mobility of birds (departures to and arrivals from foraging sites). A reduced frequency of high-intensity aggressive displays in the period when the largest number of birds was present in the colony was likely to be adaptive.