Abstract: This report deals with an investigation about the agent which causes the muscular hemorrhage of the grass carp. The disease is the most serious menace to the grass carp culture and causes much losses among both fry and fingerlings. The yearlings may also be infeeted but the mortality appears to be less. The epidemic occurs primari-ly during the warm summer months at a water temperature of 25 to 30℃. The clinical signs of diseased fish are hemorrhages.with some or all of the following signs: exophthalmia. hemorrhagic or else pale gill, hemorrhagic areas at the base of fins and gill covers. Internally, hemorrhagic areas may occur throughout the musculature. mouth cavity, intestinal tract, liver, spleen and kidney. The agent isolated from the hemorrhage of grass carp could pass through Scitz(EKS 2 asbestos) filter. The filtrates thus obtained were inoculated into the healthy fingerlings of grass carp, and on the 4th passage they still retained their infeetivity to cause typical hemorrhagic symptoms, and cytopathic effeet (CPE) has also been observed on monolayer cells of muscle and gonad of grass carp. Marked cytopathic cffeet usually appeared three to four days after inoculation at 28℃. In addition, several characteristics have been tested for this particular agent. It is relatively heat stable but sensitive to ether; its virulence can be maintained in 50% glyeerol ; and it is insensitive to tetracycline antibiotics. From these results it seems justified to infer that the agent responsible for hemorrhage of grass carp is a virus.