Abstract.Ultrastructural observations on a possible aetiological agent and tissue changes in a previously described disease of hatchery reared larval oystersCrassostrea gigasThünberg are reported. Spherical bodies, implicated in gastro‐intestinal impaction, show a strong resemblance to the marine fungus,Hyalochlorella marina. Similarities include a thick outer wall structure, plate‐like mitochondria and dense cytoplasm with small vacuoles.The dense fascicular masses, previously observed histologically at epithelial surfaces, consist of accumulations of irregular electron opaque bodies. Large accumulations of these bodies appear to exacerbate tissue damage and prevent normal velar function. In comparison with the reported virus‐like particles in this disease, these dense inclusions and associated lesions are more frequently observed and thus seem to be more important in the disease process.Notes on normal epithelial surfaces of larvalCrassostreaspp. are in