Infectivity of the infectious hematopoietic necrosis virus (IHNV) was measured by molecular filtration method. Before filtration, beef extract was added to the water (0.01%) and the condenced water was sterilized with 0.45μm filter pretreated with 1% FBS. Virus infectivity of IHNV in non-irradiated river water was 0.56TCID50/l and U.V, irradiated water was less than 0.32TCID50/l. Infectivity of the IHNV in the pond water where there was an IHN outbreak, was measured 5.60TCID50/l. Two groups of 1, 000 rainbow trout Oncorhynchus mykiss (body weight 0.11g), were cultured in 45l of aquarium, to use the river water and U.V. irradiated river water. U.V. dosage was 6.0 to 10.0×103μW⋅sec/cm2. Cumulative mortality of these 2 groups of the fish observed 25 days as 99.7% in river water and 3.9% in U.V. irradiated river water
