Comparison of N and C concentrations in bodies and shells of molluscs from the Zegrzyński Reservoir as well as an assessment of nitrogen, carbon and calcium accumulation in molluscs living in the reservoir and spatial variability of these values were performed. Interspecific differences of nitrogen and carbon concentrations were found, whereas calcium percentage in shells was similar. The amounts of the investi-gated elements accumulated in molluscs per 1m² varied within the reservoir. The total amounts of carbon, nitrogen and calcium stored in molluscs were comparable to those contained in the water column, but much smaller than those contained in surficial sediments.