| Summary: | A case of severe mortality in farmed <i>Labeo rohita</i> was investigated to characterize the causative agent. We identified the bacterial strain as <i>Aeromonas veronii</i> isolated from the gut of infected <i>L. rohita</i> by biochemical assay, scanning electron microscopy and 16S rRNA gene sequence analysis. The in vivo challenge experiment showed that the LD<sub>50</sub> of <i>A. veronii</i> was 2.2 × 10<sup>4</sup> CFU/fish. Virulence gene investigation revealed that the isolated <i>A. veronii</i> possesses Aerolysin, Cytotoxic enterotoxin, Serine protease, Dnase and Type III secretion system genes. The isolated strain was resistant to two antibiotics (ampicillin and dicloxacillin) while susceptible to 22 other antibiotics. The study further revealed that <i>A. veronii</i> induced both stresses along with non-specific and specific immune responses marked by elevated cortisol HSP70, HSP90 and IgM levels in the treated <i>L. rohita</i> fingerlings. Although the bacterial pathogen enhances the immune response, the negative effect on fish, including stress, and high mortality, create concern and a need for <i>A. veronii</i> management in <i>L. rohita</i> farms. The knowledge gained from this study would facilitate future research aimed at assessing the pathogenicity of <i>A. veronii</i>, with an emphasis on microbial disease management in other farmed fish species.
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