Lesions in the gills of the oyster, Ostrea edulis, from natural populations in Mont Saint-Michel Bay have been studied microscopically and in tissue culture. No evidence was found of the presence of the protozoans which have been thought to cause similar lesions in Crassostrea angulata. On the other hand, structures were found which closely resemble various stages in the life cycles of Labyrinthomyxa marina and Dermocystidium marinum, fungi which are thought to be responsible for mortalities of Crassostrea virginica in the USA.