A disease with mortality was observed in Japanese black abalone (Nordotis discus discus; 3+ -year old) reared at Kobe city sea-farming center in 1997. The diseased abalone showed a characteristic clinical sign of raising shell and died. There was no parasite in the gill or peduncle tissue. No significant microorganism was isolated from the peduncle tissue by bacteriological media. The disease sign was produced in healthy abalone by cohabitation with diseased abalone, but the disease was not reproduced by immersing 0.45μm-filtered homogenate of disease abalone. Histopathological examinations revealed that abundant heteromorphic cells were scattering in the muscular tissue of diseased abalone. In these heteromorphic cells, a lot of coccus like fine granules, negatively stained with Gram stain were observed. From these findings, it was considered that this case should be an infectious disease and the causative agent might be a kind of intracellular microorganism observed as the fine granules in the heteromorphic cells.