Chemical tracers (e.g., stable isotopes or trace metals) are increasingly used to study trophic ecology and feeding habits of marine organisms, while different factors can affect their values. In this context, the aim of this study was to provide information on ontogenic effects on stable carbon and nitrogen values (d13C and d15N), and on cadmium (Cd) and mercury (Hg) concentrations in several cephalopod species from the Bay of Biscay (North-East Atlantic Ocean). To this end, individuals belonging to 3 species showing a wide range of sizes were analysed for muscle d13C and d15N values, muscle Hg and digestive gland Cd concentrations. Results showed that stable isotope ratios allowed discriminating specific feeding strategies during ontogenesis. Segregation between 5 cephalopod species in terms of trophic ecology was also evidenced (different isotopic niches). By contrast, Hg concentrations varied over the same order of magnitude in these 5 cephalopod species, despite higher levels in the benthic octopus Eledone cirrhosa. Consistently, Hg concentrations followed the same ontogenic pattern, as they increased with increasing body size/age of cephalopods. Finally, Cd concentrations varied over 3 orders of magnitude among the 5 species. Despite possible effects of physiology in terms of metal bioaccumulation, Cd concentrations were likely to reflect specific feeding preferences or feeding zones and ontogenic-related variability within a single species. Thus, ontogenetic effects have to be taken into account when stable isotope ratios of carbon and nitrogen or trace metals are used as ecological tracers, the best being to focus on a given class age.