Analysis of the diet of trawl-caught hake (Merluccius merluccius) from three locations in the Bay of Biscay and the Celtic Sea in autumn 2001 showed that small hake fed almost exclusively on crustaceans (mainly euphausiids), but that there was a significant shift towards a fully piscivorous diet in hake > 23 cm. A change in fish prey was also size-dependent, because smaller hake (< 30 cm) preyed on small pelagic fish (3-12 cm), such as horse mackerel, anchovy, and pilchard, and larger hake on larger demersal prey (12-23 cm), such as blue whiting. There was a significant positive relationship between hake and fish prey length. In terms of fish prey selectivity, hake exhibited particular preference for small pelagic prey (anchovy, pilchard, and argentine) and for other hake. The diet did not generally reflect fish prey availability. Although horse mackerel and blue whiting were the two most abundant fish prey species in the environment, they were not positively selected by hake. Cannibalism accounted for a non-negligible part of the diet and was observed mainly in large hake (> 30 cm). For all sizes analysed, conspecifics constituted 19.2%W of the diet and the frequency of occurrence of hake in the stomachs was 10.53%. Most hake prey were 0-group juveniles (< 20 cm). Hake cannibalism appeared to be influenced mainly by the abundance of juveniles and the overlap between distribution patterns of juveniles and adults.