Comparisons of P concentration in soft tissues and shells of molluscs being main components of malacocenoses of the Zegrzy.ski Reservoir as well as analyses of spatial, temporal and size dependent diversity of P concentration in particular mollusc species were performed. Percent content of P in the soft tissues of the investigated species varied significantly when comparing taxonomically distant species, whereas P level in shells was relatively similar. Owing to the interspecies diversity and size dependence of P concentration, the species composition and dominance patterns of malacocenoses as well as size-frequency relations in populations of dominant species are expected to influence the accumulation of P by molluscs in a given habitat.