An entire bed of the blue mussel Mytilus edulis, consisting of 5,000 individuals/cu.m., died during June, 1971 in the effluent canal of a steam generating plant when the temperature increased above 27C. Similarly, the population in the intake canal disappeared when temperatures rose above 27C in August. Laboratory studies showed that M. edulis could not tolerate continuous temperatures above 27C, and feeding stopped shortly after the mussels were exposed to 25C. Histopathological studies indicated that the cause of death of this bivalve was associated with degeneration of the frontal and laterofrontal cilia of the columnar epithelium of the gill filaments. In conjunction with this, there was necrosis and sloughing of the epithelium of the intestinal diverticula. Extensive amoebocytic infiltration was noted in the byssogenous cavity, gill filaments and stomach wall.