ABSTRACT: Carbon and nitrogen isotopic ratios in the body of the bivalves Ruditapes philippinarum and Mactra veneriformis were measured to estimate the role of the bivalves in the ecosystem in an estuary. The isotopic signatures were compared with those obtained from particulate organic matter (POM) in the middle reach of the river, in the estuary, and off the estuary. Both the bivalves showed constant δ13C and δ15N values of approximately −17 and 10 that were significantly enriched compared with those for POM in the habitat (−25 and 4). The relative contributions of terrestrial organic matter to the POM in the habitat and to the bivalve diet were estimated: the former was approximately 90% while the latter approximately 10%. This indicates that the bivalves select marine POM from the organic matter available in their habitat, and play a minor role in the direct removal of terrestrial particulate matter. Temporal changes in δ13C and δ15N for both bivalves were small, but a distinct decrease in δ13C was observed following a period of heavy rain. During rainfall the concentration of terrestrial material increased in the habitat, and the bivalves would then have to change their food sources temporarily.