Diet of Great Cormorant (Phalacrocorax carbo sinensis) nestlings was studied during four consecutive breeding seasons (1999-2002) at three Greek colonies (Axios Delta, and Lakes Kerkini and Mikri Prespa) in order to assess variation and commercial value of prey. A variety of fish taxa were found in nestlings’ regurgitates in each area and season, but only one or two dominated by numbers or biomass. Black Goby (Gobius jozo), Round Sardinella (Sardinella aurita) and Twaite Shad (Alosa fallax) were the most important prey in the Axios Delta; Bleak (Alburnus alburnus), Giebel (Carassius auratus gibelio) and Roach (Rutilus rutilus) at Lake Kerkini; Chalcalburnus belvica and Giebel at Lake Mikri Prespa. Nestling diet varied both seasonally (but only at Lake Kerkini significantly so) and annually (significantly in the Axios Delta and at Lake Mikri Prespa). Temporal changes can be attributed to changes in prey availability and abundance and confirm this bird’s opportunistic behavior. Between-colonies, differences in diet were significant, probably due to differences in habitat and prey species diversity and composition. The low consumption of valuable fish prey by Great Cormorant nestlings (<10%, numbers and biomass) suggests minimal competition with human interests.