Abstract: Electron microscopy of ultrathin sections were conducted in liver, spleen and heart of the mandarin fish (Siniperca chuatsi Basilewsky) with outbreak of acute lethal disease. Spherical virus particles of 280um in diameter were found distributed in cytoplasm, nuclei and gaps of the cells. The particles were isolated from tissues of the infected fish. When the filtered tissue extracts from diseased fish were inoculated onto culturing confluent cells, cytopathogeic effects were induced in the infected cells.Health fish was experimetally infected by the tissue extract from diseased fish or amplificated virus extracts from culture cells, and lethal disease of the fish was resulted. Anatomical and histopatholgical studies were performed in gill, liver, spleen, heart and kidney from naturally and antificially diseased fishes infected by the virus, and were compared and analysed with same organs of health individuals. It is supported that the virus found from the fish siniperca chuatsi is the lethal pathogen of the infective disease of the fish Siniperca chuatsi.