Abstract: We isolated and identified the pathogen causing disease in largemouth bass (Micropterus salmoides) from a farm in Xinxiang, and investigated its pathogenicity. We combined the annotation of drug-resistant genes to analyze drug sensitivity results, and compared its genome structure, virulence factors and closely related strains to search for the common immune protective antigen functional genes. By the analyses of colony morphology and physicochemical properties, and based on the identification of isolated bacteria through 16S rRNA gene sequence alignment, we conducted hemolysis test, artificial regression infection test, drug sensitivity test, and whole genome sequencing analysis, then identified the isolated bacteria XXLX2 which was identified as Nocardia seriolae, without hemolysis circle on blood agar plates. The median lethal dose (LD₅₀) for largemouth bass was 1.49×10⁵ CFU·mL⁻¹ through regression infection tests, and the symptoms of infected bass were consistent with those of naturally diseased bass. The XXLX2 strain exhibited resistance to polymyxin B, erythromycin, and β-lactam antibiotics, and the results of drug resistance gene and drug sensitivity test were generally consistent. Through genome comparison analysis, the XXLX2 strain showed close phylogenetic relationship and good collinearity with three strains of N. seriolae from different sources, namely EM150506, NK201610020, and UTF1. There were certain differences in the comparison of virulence factors between the XXLX2 strain and the three strains of N. seriolae, but generally, they exhibited high conservation. Genomic annotation and comparative analysis of the XXLX2 strain provide basic data support for further exploration of the pathogenic mechanism of N. seriolae and research on genetically engineered vaccines.