ABSTRACT: Stable isotope ratios of nitrogen and carbon were determined in bone collagen, egg proteins and keratin from epidermal carapace scutes of loggerhead Caretta caretta, green Chelonia mydas, and leatherback Dermochelys coriacea turtlesfrom the Mediterranean Sea and the European Atlantic Ocean. Isotope ratios in proteins from loggerhead turtles indicate that this species occupies a higher trophic position than green turtles. Leatherback turtles are apparently intermediate in trophicstatus. Within loggerhead turtles, both isotope ratios correlated positively with body size, indicating a trend of increasing trophic level with age. Within the size range of green turtles sampled, there was apparently no change in trophic level inrelation to body size. In absolute terms, isotope signatures of egg proteins were markedly different from those of bone collagen. However, inter-specific differences in isotope values were consistent with those in bone protein. The novel application ofthis technique to marine turtles generally confirmed dietary information collected conventionally in these species and has also afforded additional insights into shifts in diet and trophic status as turtles mature.