Abstract: Two Aeromonas sobria isolates, isolated from cultured Carassius auratus gibelio during an outbreak of mass mortality in Yancheng city of China, were identified to the species level by phylogenetic analysis based on gyrB gene and biochemical tests. The isolates were identified to be the pathogeny of Red-Gill through investigating for the virulence of bacteria by recurrent tests and histopathology. We also described the macroscopic and microscopic lesions in Carassius auratus gibelio naturally infected with A. sobria. Our findings confirmed that the 2 isolates, rather than cyprinid herpesvirus 2, were the primary pathogen of Red-Gill disease and damaged the immune system in Carassius auratus gibelio. The results of drug sensitivity tests showed that the bacterial isolates was sensitive to ofloxacin, chloromycetin and norfloxacin; While, resistant to oxacillin, vancomycin, ampicillin, penicillin, clindamycin etc. Together, our findings demonstrate the pathogenesis of Aeromonas sobria in Carassius auratus gibelio and establish the foundation for Red-Gill in cultured Carassius auratus gibelio.