Stable isotope data are often used to assess diet, trophic level, trophic niche width and the extent of omnivory. Notwithstanding ongoing discussions about the value of these approaches, variations in isotopic signatures among individuals depend on inherent variability as well as differences in feeding habitats. Remarkably, the relative contributions of diet variation and inherent variability to differences in d15N and d13C among individuals have not been quantified for the same species at the same life history stages, and inherent variability has been ignored or assumed. We quantified inherent variability in d13C and d15N among individuals of a marine fish (the European sea bass, Dicentrarchus labrax) reared in a controlled environment on a diet of constant isotopic composition and compared it with variability in d13C and d15N among individuals from wild bass populations. The analysis showed that inherent variability among reared individuals on a controlled diet was equivalent to a large proportion of the observed variability among wild individuals and, therefore, that inherent variability should be measured to establish baseline variability in wild populations before any assumptions are made about the influence of diet. Given that inherent variability is known to be dependent on species, life history stage and the environment, our results show that it should be quantified on a case-by-case basis if diet studies are intended to provide absolute assessments of dietary habits.