Filter-feeding mussels historically comprised most of benthic biomass in many streams. They contribute to stream ecosystem functioning by linking the water column and benthic habitats. Both native and nonnative species coexist in many streams, but their ecological roles are not well quantified. The invasive bivalve, Corbicula fluminea, has the potential to alter profoundly organic matter dynamics and nutrient cycling in streams. We compared stable isotope ratios and tissue and biodeposit stoichiometry of the native freshwater mussel, Elliptio crassidens, and C. fluminea in a Coastal Plain stream (Ichawaynochaway Creek, a tributary to the lower Flint River, Georgia, USA) to assess their trophic niche space and potential effects on nutrient cycling. We hypothesized that C. fluminea would assimilate a larger range of materials than E. crassidens. To determine dietary overlap of C. fluminea and E. crassidens, we measured the elemental and stable isotopic compositions (δ¹³C and δ¹⁵N) of their tissue. Corbicula fluminea showed lower trophic fidelity than E. crassidens and was able to acquire and assimilate a wide range of resources, as illustrated by their wide range of δ¹³C values. Corbicula fluminea also might alter nutrient cycling in the benthic environment of streams because they retain less N than E. crassidens, as reflected by their higher tissue C∶N. In the laboratory, we measured C and N in biodeposits (feces and pseudofeces) from the 2 species. Corbicula fluminea released more N through their biodeposits relative to E. crassidens by mass, a result implying that C. fluminea might modify nutrient cycling in streams. Our results show important differences in the food resources assimilated and the nutrients deposited as feces and pseudofeces by these 2 bivalves. Furthermore, our results demonstrate how invasive species, such as C. fluminea, can alter aquatic environments through differences in species traits within a functional group.