| Summary: | <i>Aeromonas veronii</i> is as an important opportunist pathogen of many aquatic animals, which is wildly distributed in various aquatic environments. In this study, a dominant bacterium GJL1 isolated from diseased <i>M. salmoides</i> was identified as <i>A. veronii</i> according to the morphological, physiological, and biochemical characteristics, as well as molecular identification. Detection of the virulence genes showed the isolate GJL1 carried outer membrane protein A (<i>ompA</i>), flagellin (<i>flgA</i>, <i>flgM</i>, <i>flgN</i>), aerolysin (<i>aer</i>), cytolytic enterotoxin (<i>act</i>), DNases (<i>exu</i>), and hemolysin (<i>hly</i>), and the isolate GJL1 also produced caseinase, lipase, gelatinase, and hemolysin. The virulence of strain GJL1 was confirmed by experimental infection; the median lethal dosage (LD<sub>50</sub>) of the GJL1 for largemouth bass was 3.6 × 10<sup>5</sup> CFU/mL, and histopathological analysis revealed that the isolate could cause obvious inflammatory responses in <i>M. salmoides.</i> Additionally, the immune-related gene expression in <i>M. salmoides</i> was evaluated, and the results showed that <i>IgM</i>, <i>HIF-1α</i>, <i>Hep-1</i>, <i>IL-15</i>, <i>TGF-β1,</i> and <i>Cas-3</i> were significantly upregulated after <i>A. veronii</i> infection. Our results indicated that <i>A. veronii</i> was an etiological agent causing the mass mortality of <i>M. salmoides</i>, which contributes to understanding the immune response of <i>M. salmoides</i> against <i>A. veronii</i> infection.
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