Abstract: In April 2018, a large number of largemouth bass (Micropterus salmoides) were infected with lethal sarcoidosis in a farm in Qionglai, Sichuan Province. In order to define the potential pathogens, a bacterial strain named HSY-NS02 was isolated from skin ulcers, effusions from fish swimming bladder’s cavity and liver of the diseased M. salmoides by traditional pathogen isolation methods. The strain HSY-NS02 was identified as Nocardia seriolea by means of morphologic structure and staining observation, PCR amplification and sequence analysis of 16S rRNA gene and specific primer, constructing phylogenetic tree, physiological and biochemical test. Furthermore, the pathogenicity of the strain HSY-NS02 was confirmed by the infection experiment in healthy M. salmoides. The results showed that the strain HSY-NS02 was cofirmed to be pathogenic to healthy M. salmoides and the same bacterium could be recovered from these infected M. salmoides. Histopathological observation and analysis of diseased M. salmoides showed that there were different degrees of chronic granulomatous lesions in skin ulcers, heart, liver, spleen, kidney and gill, of which spleen lesions were the most serious. The antibiotic susceptibility of the strain HSY-NS02 was carried out to guide clinically choosing medicine, and the results showed that the strain was sensitive to gentamicin, neomycin and nystatin, but resistant to other 18 kinds of antibiotics, which implied the strain HSY-NS02 was resistant to multiple antibiotics.