The diet of young Atlantic bluefin tuna (Thunnus thynnus) in the Bay of Biscay foraging ground was assessed using stomach-content analysis (SCA) and stable-isotope analysis coupled with isotope mixing model. Whereas SCA showed that the young tuna fed mainly on horse mackerel (Trachurus trachurus), the estimation of liver and muscle mixing model analyses indicated that northern krill (Meganyctiphanes norvegica) was the most important food resource. These contrasting observations are probably due to the different time scales spanned by the different methods. The present results suggest that young bluefin tuna prey at different trophic levels of the Bay of Biscay pelagic food web, thus behaving as an opportunist and generalist predator.