Abstract: Rice field eel, Monopterus albus, freshwater fish, belongs to the synbranchidae. It is an economically important species for fish production in China, especially in southern of China. The diseases of cultured rice field eel have been the bottle-neck for the sustainable development of rice field eel cultivation. The muscular ulcer disease is a kind of common disease to rice field eel and is suggested to be one of the most important diseases in industrial artificial-breeding. In 2007, the outbreak of muscular ulcer disease in the rice field eel farmed in Sichuan province, whichwas high incidence and mortality and caused lots of economical loss to the farmer. To make certain the causative pathogen, we studied the etiology and virulence of rice field eel so as to provide basic data for the farming industry of rice field eel. In the present study, we used the morphological, physiological and biochemical methods to identify the isolated bacterium. Meanwhile, to ensure the results accurately, the phylogenetic analyses were also used. Liver, kidney and spleen of diseased rice field eel were aseptically streaked on Nutrient Agar by swabbing. After incubation, pure cultured colonies were obtained and one causative bacteria (hs-1) was isolated from liver, kidney and spleen of moribund rice field eel. The characteristics of causative bacteria were tested by bacteria micro amount biochemical fermentation tube, Gram's staining and another method. The phylogenetic comparison based on 16S rRNA gene of the causative bacterium was also studied, the amplified nucleotide sequence of isolated bacterium (GenBank accession No. FJ461353) had 99% homology with the other 16S rRNA sequences of A.Sobria. Based on the morphological, physiological and biochemical characteristics, as well as the results of phylogenetic analysis, the causative bacterium was conducted as A.Sobria. Pathogenicity tests in rice field eel showed that the symptoms of rice field eel were similar with the natural diseased eels. The mortality of high dose injected group (3.0?08 CFU) was 100%. In all injected groups, the similar symptoms appeared in 48h post-artificial injection. The same pathogen was isolated from diseased fish caused by the artificial infection. The mortality of loach and carp in Pathogencity tests were 80% and 100%, which showed that the isolated strain also had pathogenicity to the others fish. A.Sobria was typically opportunistic pathogen and secondary invader. The strain in present study was resistant to much commonly antibiotics, such as Ampicillin, Acetylspiramycin, Novobiocin, and so on. The results of drug sensitivity test also showed that Cefotaxime Sodium, Rocephin, Norfloxacin, Ciprofloxacin, Lomefloxacin, Trimethoprim and Norfloxacin were most effective against the pathogenic bacterium.However, the treatment of A. hydrophila infections would be painstakingly tough and expensive. Therefore, the farmers should pay more attentions to cultural management, aquatic techniques and regulation.