The diet of the Great Cormorant (Phalacrororax carbo) was studied on the Finnish side of Gulf of Finland, Baltic Sea, by collecting regurgitated fish in a colony. Most abundant species of fish were Viviparous Blenny (Zoarces viviparus), Roach (Rutilus rutilus) and Perch (Perca fluviatilis), most of which are economically unimportant. Cormorants switched their diet during the breeding season. Before young were hatched, cormorants ate mainly Roach and Perch; when young were less than 25 days, old they ate mainly small Viviparous Blennies. Later, adults provided larger Roach and Perch for their young. The results suggest that several fish-sampling visits are needed to record the food captured by cormorants during the breeding season. Few samples restricted in time may bias the composition of fish species and size of the fish in diet.