Carbon and nitrogen stable isotope ratios ([delta].sup.13C and [delta].sup.15N) of sympatric nereidid polychaetes including Hediste diadroma, H. atoka, Tylorrhynchus osawai, and Simplisetia erythraeensis were examined at six stations along the Japanese coast to reveal the trophic niches of nereidid polychaetes in estuarine benthic food webs. The species studied exhibited a large degree of spatial variation in [delta].sup.13C values (- 25.3 to - 13.7â°) among stations, suggesting site-based differences in nereidid dietary components. The low [delta].sup.13C values of nereidids suggest that terrestrial plant materials are important components of their diets in river-affected estuarine habitats, which were also shown by a [delta].sup.13C-based stable isotope mixing model (SIAR). [delta].sup.13C values of H. diadroma and H. atoka were 0.3-3.1â° lower than those of the sympatric T. osawai and S. erythraeensis, while no significant difference was found between Hediste species. A biochemical assay revealed cellulase activity in H. diadroma, H. atoka, and T. osawai (S. erythraeensis was not examined), while the apparent molecular size of cellulase differed between the two genera. These observations suggest that some nereidids can directly assimilate terrestrial plant matters using cellulases, although the dietary importance of terrestrial matter varies among genera. Species with the ability to digest cellulose may be better able to colonize river-dominated estuarine habitats, where refractory terrestrial detritus is a major carbon source.