Maximum dive depths, timing and duration of foraging trips and diet of the guanay cormorant Phalacrocorax bougainvillii were investigated in January/February 1995 and February 1996 at the headland of Punta San Juan, Peru. Results from 27 birds engaged in chick rearing showed that the mean maximum dive depth was 33.9 ± 2.6 m, with a deepest dive of 74 m. A large flock of non-breeding guanay cormorants (about 140 000 birds) foraged only during daylight, with modal departure and arrival times of 09:00 and 16:00 respectively. Arrival times were more variable than departure times. Cormorants flew almost exclusively parallel to the coast (92% of cases), with feeding frenzies observed mainly within 1-3 km of the coast. Duration of foraging trips averaged 6.2 h (CV = 34%), and increased significantly throughout the season. There were interannual differences in diet composition, but generally Peruvian silverside Odonthestes regia regia, Peruvian anchovy Engraulis ringens and mote sculpin Normanichthys crockeri were the main food delivered to cormorant chicks, accounting for 45, 29 and 16% of the overall number of items respectively. Maximum dive depths suggest that guanay cormorants not only feed just below the surface, but that they can also exploit much of the water column. Short-term variations in the timing and duration of foraging trips of guanay cormorants probably reflect the unpredictable and patchy distribution of their main prey.