During October, 2009, a serious infectious disease began to break out in cultured largemouth bass() of Foshan area in Guangdong province of China. Affected fish had following clinical signs including slowly swimming, floating at the surface of water, pale gill, swollen liver, spleen and kidney, pale liver and dark red spleen. Transmission electron microscopy of the focus liver, spleen and kidney revealed numerous cytoplasmic, icosahedral virion without envelope. The diameter of the virion was about 156−160 nm. Filtrate dilutions of liver, spleen and kidney were inoculated onto the monolayers of EPC, FHM and CIK cells respectively, but cytopathic effect(CPE) was not observed at the 8th days after infection. According to known gene sequences of iridovirus, special primers were designed to amplify the gene of the current virus. Liver, spleen and kidney DNA was extracted as PCR templates. PCR products were sequenced and the nucleic acid sequence of viral MCP gene is 98% identical to those of ISKNV (Infectious spleen and kidney necrosis virus) and DGIV (Dwarf gourami iridovirus), and the amino acid sequence is 99% to those of them. Transmission electron microscopy and gene sequence of the virus indicated that the pathogen is a virus of genus , family Iridoviridae. Experimentally infection for healthy largemouth bass with filtrates of tissues by intramuscular injecting caused a more than 90% mortality and the signs of the infected fish were similar to those of natural infection. In contrast, control fish exhibited no mortality and no clinical signs of infection. LMBV was a ranavirus isolated from diseased largemouth bass in USA. Naturally infected largemouth bass with LMBV appeared normal and lacked external lesions, but they floated at water’s surface with an enlarge swim bladder. A similar ranavirus was isolated in China in 2008, and its predicted amino acid sequence was 98% identical to corresponding sequence of LMBV, and naturally infected largemouth bass with the virus were all characterized by extensive ulceration in skin and muscle. Different from the symptoms of the above viruses, largemouth bass infected by the currently megalocytivirus exhibited swollen liver, spleen and kidney. All of the three viruses could cause death of largemouth bass, but the clinical signs of the infections were different. This study is the first report on largemouth bass naturally infected by megalocytivirus