Digestive glands of Ostrea edulis affected by an epizootic have been examined by light- and electron- microscopy. Round, compact lesions in the digestive gland cells contain refractive elements staining slightly blue or red with Geimsa. Ultra-structurally, each lesion contains (a) dense cytoplasma, without mitochondria and with paracrystal-line inclusions, (b) enclosed regions with vacuolar cytoplasm, no mitochondria and two types of formation; one is opaque, the other a vacuole containing ribosomes, lamellar structures and peripheral ovoid particles. The nature and significance of these structures is not known.