Sudden outbreak of a disease caused by the blood fluke Paradentacylix occurred in May, 1993 among net-cage cultured juvenile amberjack, Seriola dumerili which had been imported from Hainan, China, a few months before the onset of the disease. The cumulative mortality rate ranged from 50 to more than 80% in one month. Dead fish were characterized by opened mouth and opercula, showing typical sign of suffocation. Parasitological examination revealed that eggs of P. grandispinus and/or P. kampachi were accumulated in the afferent arteries of the gills of all dead fish. Since no other pathogens were detected, it was concluded that the mass mortality was caused by these blood flukes. It could not be specified whether the cercarial invasion had occurred prior to or after the introduction of the fish into the Japanese waters.
