1. Stable isotope data are widely used to track the origins and transformations of materials in food webs. Reliable interpretation of these data requires knowledge of the factors influencing isotopic fractionation between diet and consumer. For practical reasons, isotopic fractionation is often assumed to be constant but, in reality, a range of factors may affect fractionation. 2. To investigate effects of temperature and feeding rate on fractionation of carbon and nitrogen stable isotopes in a marine predator, we reared European sea bass Dicentrarchus labrax on identical diets at 11 and 16°C on three ration levels for 600 days. 3. Nitrogen trophic fractionation (?d15N) was affected by temperature. Bass ?d15N was 4.41‰ at 11°C and 3.78‰ at 16°C. 4. Carbon fractionation (?d13C) was also affected by temperature. Bass ?d13C was 1.18‰ at 11°C and 1.64‰ at 16°C. The higher lipid content in the tissues of bass reared at cooler temperatures accounted for the temperature effect on ?d13C. When ?d13C was determined using mathematically defatted values, there was a direct effect of ration size and ?d13C was 2.51, 2.39 and 2.31‰ for high, medium and low rations, respectively. 5. Reported ?d15N for all treatments exceeded the mean of 3.4‰ widely used in ecological studies of fish populations and communities. This would confound the interpretation of d15N as an indicator of trophic level when comparing populations that are exposed to different temperatures. 6. The ?d13C of 0-1‰ commonly applied in food web studies did not hold under any of the temperature or feeding regimes considered and a value of 2‰ would be more appropriate.