The diet of the smooth-hound, Mustelus mustelus, in the eastern Adriatic Sea was investigated with respect to fish size and season. Stomach contents of 139 specimens, 67 to 137 cm in TL, collected by bottom-trawl from 2001 to 2003 were analyzed. Of the total number of stomachs examined, 24 were empty (17.2%). The prey items identified in stomachs belong to eight major groups: Cephalopoda, Polychaeta, Nemertina, Stomatopoda, Decapoda, Enteropneusta, Tunicata and Teleostei. Decapod crustaceans were the most abundant prey (%IRI = 63.1) especially for individuals up to 110 cm TL. Teleosts were second in importance (%IRI = 31.2), whereas other prey groups were of minor importance and were probably incidentally ingested food. At the species level, the decapods Atelecyclus rotundatus (%IRI = 6.7) and Munida rugosa (%IRI = 4.3) followed by the teleost Sardina pilchardus (%IRI = 2.7) were the most frequent prey.With M. mustelus growth, the proportion of decapods decreased whereas that of teleosts and cephalopods increased. Diet composition showed little seasonal variation; decapods were the most important prey taxon in all seasons, especially in summer. The results indicate that the smooth-hound feeds on a wide range of prey items and can be considered an opportunistic predator.