Soft tissue δ13C values were determined in vestimentiferan tube worms, alvinellid polychaetes and molluscs from Axial Seamount and Middle Valley, North-east Pacific. Inorganic carbon in mollusc shells and water samples was also analysed. In the vestimentiferan, Ridgeia piscesae, which lives in symbiosis with sulphur-oxidizing chemolithoautotrophic bacteria, tissue samples from the Axial vents showed δ13C values from −11 to −16‰, whereas at Middle Valley, where venting occurs through sediments, the δ13C ranged from −16 to −26‰. The tissues of an associated polychaete, Paralvinella palmiformis, which feeds on free-living bacteria, had δ13C values in the range −21 to −26‰. The bivalve Calyptogena from Middle Valley was more depleted than Ridgeia and Paralvinella, −37‰, closer to the ratios found in chemolithoautotrophic symbioses in non-vent habitats. Considerable, but variable, depletion (−23 to −42‰) was found in small gastropods. Mollusc shells and diluted vent water differed little in δ13C compared to inorganic carbon in ambient deep sea-water.