The nutritional role of silver for the freshwater crustacean, Daphnia magna, was examined through four generations of deprivation. Silver inclusion in animal media was set at a nominal zero (employing chemicals of the highest available purity). Both reproduction (-60%) and life span (-40%) were negatively affected when compared to animals reared in the presence of 0.4 ng g(-1) Ag. These results strongly suggest a nutritional requirement of silver for daphnids at nanomolar concentrations.