A mass mortality of the deep sea scallop, Plaeopeeten magellanieus, which occurred in the autumn and winter of 1979-80, in Narragansett Bay, Rhode Island was studied. Grossly the animals possessed greyish, flaccid adductor muscles and histologically they showed myodegeneration. Eighty-eight per cent of the 34 animals examined were infected with intracellular prokaryotes on the gills, plicate membranes and other epithelial surfaces of the body. The morphology of this prokaryotic organism is suggestive of rickettsia. Heavy infection was positively correlated with extensive myodegeneration.

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