To investigate the cause of an outbreak of unknown disease occurred in cultured largemouth bass (Micropterus salmoides) in a reservoir in Fujian Province, with a cumulative mortality of over 30% in March 2021, the dominant bacterial colonies were isolated from the liver, spleen and kidney tissues of affected fish, the pathogenic bacteria was identified by artificial infection experiments, and its species was identified by physicochemical characteristics, 16S rDNA sequences, and mass spectrometry analyses. Further, virulence gene detection, drug sensitivity research and histopathological observation were carried out. A predominant bacteria strain Yersinia ruckeri was isolated from the spleen of the inffected M. salmoides. A set of virulence genes including YrP1⁺ - YhlA^{+ -} YhlB⁺ - invF^- - traC^- - traL^- - traN^- were detected by PCR from in the isolated strain, with a LD₅₀ of 3.8×10⁵ CFU per fish by intraperitoneal injection. The antibiotic sensitivity test showed that the pathogen was sensitive to enrofloxacin, doxycycline hydrochloride, flumequine, neomycin sulfate and florfenicol. Histopathological observation found that Y. ruckeri infection caused significant damages to liver, spleen, kidney and other organs of M. salmoides. The main pathologic lesions showed obvious degeneration, necrosis and infiltration of the inflammation cells. It could be included that the current research is the first report of cultured M. salmoides infected by Y. ruckeri, which will be helpful for the early diagnosis and better prevention of the disease in cultured M. psalmodies.