The gut contents of 90 individuals of common dragonet Callionymus lyra were analysed, of which 76 contained prey, along with 100 corresponding benthic grab samples in order to assess the diet of C. lyra in relation to the availability of its prey in the environment. Forty-five prey taxa were identified in the diet from 350 potential prey taxa identified in the environment. Calculation of the index of relative importance (IRI) found the main food components were crustaceans (%IRI = 86·3), mostly the porcelain crab Pisidia longicornis (%IRI = 43) and other decapods (%IRI = 18). Polychaetes played only a supplementary role in the overall diet (%IRI = 12·5). This study demonstrated that C. lyra is predominantly an opportunistic feeder that can modify its feeding behaviour to exploit alternative, more abundant prey.