Recovery of the federally endangered oyster mussel (Epioblasma capsaeformis) depends upon present efforts to successfully propagate and rear juveniles, and hold adults in a captive environment. An understanding of food quantity and food quality requirements for E. capsaeformis is vital to successful captive care. Neither an optimum food quantity nor specific food quality requirements have been identified for adults of this species. Oyster mussels were collected from Clinch River, Tennessee, and held at White Sulphur Springs National Fish Hatchery, West Virginia, during spring, summer, and fall 2005 and 2006. An optimum feed ration for adults fed green-algae (Neochloris oleoabundans) was determined via measurements of filtration rate and absorption efficiency. Diet quality was examined by targeting protein demand of specimens, since protein is essential for all biosynthesis and serves as a secondary energy source. Seasonal protein demand was examined by feeding mussels diets of N. oleoabundans differing in protein composition, and consequently C/N ratio. Mussels were fed a diet that was low, intermediate, or high in protein for three weeks in spring, summer, and fall. O/N ratio was used to evaluate nutritional status of mussels before and after trial diets. Regression analyses of C/N and O/N ratios provided an indicator of seasonal protein demand for adults of this species.