Abstract: An outbreak of an infectious disease occurred amongst farmed Oncorhynchus mykiss at Penzhou city, Sichuan Province, in October 2017 with heavy mortality of 90%. Major symptoms of the diseased fish included darkening of the body, yellow mucoid fluid fecal casts on anal. Internally, the symptoms were hemorrhage in the swim bladder and peritoneum, gastric distention, enteritis and hydropericardium. Bacteriologic test was negative. Histopathologically, the spleen showed typical coagulative necrosis with marked degeneration and necrosis in the haematopoietic tissue. Hepatocytes showed degeneration and necrosis and edema in some areas. After filtration treatment, the tissue suspension was injected intraperitoneally into 60 O. mykiss. The O. mykiss displayed similar clinical symptoms as fish that were naturally infected. The O. mykiss died acutely in the trial group (cumulative mortality rate of 85%), while there was no abnormal fish in the control group. After filtration treatment, the spleen tissue suspension was inoculated to the Fathead minnow cells (FHM). The typical cytopathic effects were observed after three blind passages in FHM. RT-PCR assay of tissue filtrates from the fish naturally infected, the fish artificially infected and diseased cells were IHNV-positive. The amplification products shared 98.2% identity to that of IHNV nucleoprotein gene at nucleotide level. Phylogenetic analysis based on the G gene showed that the viral isolate was classified into Asian isolates, and belonged to the JRt genotype.