Mass mortalities accompanied by the changing of the color of the adductor muscle to red-brown have been occurring in the cultured Japanese pearl oysters Pinctada fucata martensii in western Japan. Histopathological changes commonly appeared in the loose connective tissue of the mantle and less frequently in the adductor muscle in the affected pearl oysters of the farms. To clarify the possibility of the involvement of an infectious agent in this epizootic, two infection experiments were performed by transplantation of the mantle piece of the affected pearl oysters into healthy oysters and cohabitation of the affected and healthy pearl oysters. During the experimental period of three months, the mortality of the test group was 13.3% in the transplantation and 30.8% in the cohabitation. The color of the adductor muscle of all survivals had changed to red-brown and the pathological changes were also reproduced in both experiments. On the other hand, mortality and color changes in the adductor muscle were not ob served in the corresponding control groups. Accordingly, these results strongly suggest that the mass mortality of the cultured pearl oysters is caused by an infectious agent.