Freshwater native bivalves have declined in recent decades and are considered one of the most threatened faunistic groups worldwide. Many mechanisms can be responsible for this declining trend, being competition for resources with invasive bivalve species one example. When in sympatry with native species, invasive bivalves may be superior competitors regarding food acquisition due to more flexible diets, higher abüity to find food resources or use it with higher efficiency. However, their potential for food competition is poorly known. In this study, we aimed to evaluate the potential overlap in resource use by bivalve species (native and invasive) living in sympatry in the Tua basin (NW-Iberian Península), by identiíying and quantifying the contribution of different organic matter (OM) sources to their biomass. To accomplish this objective, the C (ÕiaC: "C/i^C) and N (Ô"N: "N/xN) stable isotope ratios of bivalves and its potential OM sources were analyzed. Special attention was given to the possible overlap between native bivalve species Anodonta anatina, Unio delphinus and Potomida littoralis, and one of the most pervasive invasive species in European aquatic ecosystems, the Asian ciam Corbicula fluminea. The stable isotope analysis suggests that C. fluminea and P. littoralis rely on "C- and "N- enriched sources such as microphytobenthos or other microbial enriched material and that the potential for resource competition is high because their trophic niche overlaps. Contrarily, the potential for competition with A. anaüna and U. delphinus is likely lower because both native species were in general more "C- and isN- depleted than C. fluminea indicating the consumption of more i3C- and "N- depleted food sources such as pelagic OM.