A largemouth bass ulcerative disease broke out in the summer and autumn of 2008 at Foshan area of Guangdong Province. The clinical signs of this disease were a great size of ulcer and necrosis on body surface and inflamed fin bases. The ulcerative disease broke out quickly and caused a high rate of mortality, which led to large losses for many largemouth bass breeding farms and threatened its aquaculture industry. So it was necessary to find out the cause of this disease and take corresponding countermeasures for prevention and cure or diagnosis. In this research, in order to identify the pathogen of largemouth bass ulcerative disease, five Aeromonas hydrophila strains were isolated from the muscle of diseased fish for artificial infection. Experimental infection under aquarium conditions suggested the infected largemouth bass did not show any clinical sign. The homogenated muscle ultrafiltrate from diseased fish with no bacteria was prepared and injected into healthly largemouth bass. 7 days later, these treated largemouth bass showed typical symptoms of ulcerative disease similar to ehose of natural diseased fish. The muscle specimens of artificially infected and naturally diseased largemouth bass were prepared and made into ultrathin sections for observing the morphological character of the pathogen. Electron microscopic observation showed that a large amount of viral particles existed in the cytoplasm. The maturated particles were enveloped and had hexagonal outline. The diameter of these virions was about 145.5 nm. A pair of primers based on the conserved region of the reported iridovirus major capsid protein（MCP）gene was designed, and a 530 bp fragment was amplified from the DNA extracted from the muscle of artificial infected largemouth bass and sequenced. The amino acid sequence showed 39.5％-100％ homology with the MCP gene sequences of other iridoviruses. Both electron microscopic observation and MCP gene sequences analysis results showed the virus belonged to the genus ranavirus of family Iridoviridae，which was different from LMBV on molecular character and induced clinical signs. These results indicated that this virus was the lethal pathogen of largemouth bass ulcerative disease. The ulcerative disease reported here was designated temporally largemouth bass ulcerative syndrome.