During summer 1985, reduced feeding, reproductive failure and massive mortalities were observed in populations of the blue mussel Mytilus edulis during an extremely dense algal bloom in Narragansett Bay, Rhode Island (USA). The dominant alga was an extremely small (1 to 2 micrometers diameter) non-flagellated chrysophycean form previously unobserved in coastal or offshore waters. Effects of the alga on clearance rates of mussels were tested using natural particulates (NBP) collected from Narragansett Bay during the bloom. Reductions in clearance rate were independent of cell size and extra-cellular exudates of the bloom alga. Experiments repeated with the hard shell clam Mercenaria mercenaria produced similar clearance rate responses. High mortality in indigenous mussels was coincident with peak bloom conditions, varying from 30 to 100% at stations along the main axis of the bay. In addition, complete reproductive failure of the population was apparent. This effect could not otherwise be explained by environmental conditions of temperature, salinity or dissolved oxygen concentration. It is concluded that starvation stress induced by noxious properties of the bloom alga in combination with seasonal effects (reproductive stage, temperature) was the cause of the demise of M. edulis.