Abstract: In August 2011, a mass fish kill of the cultured Mylopharyngod piceus occurred in a farm in Qianhuang Town, Wujin District of Changzhou City, Jiangsu Province of East China. The main symptoms of the moribund fish were mainly manifested in the hyperemia of mouth, head, eye, gill cove edge, fin and caudal peduncle, and anal swelling. The postmortem observations showed that the intestine contained no food and was red from hyperemia, there were many yellowish effusion in abdominal, the liver was purple, and the gallbladder swollen. Large amount of bacteria with growth advantage was isolated from the liver and blood of diseased M. piceus, and through artificial infection test, the isolated strain (WJ110801-1) showed very strong pathogenic to M. piceus. The phenotypic characteristics of the isolated strain, including the morphological characteristics, physiological and biochemical characteristics, virulence factor detection, extracellular enzyme analysis, and susceptibility test, were examined, and the 16S rRNA gene was amplified and compared with those sequences deposited in GenBank databases. The 16S rRNA sequence of the pathogen was more than 99% homology with that of Aeromonas hydrophila. The phenotype and molecular biological analyses show that the identified isolate belongs to A. hydrophila. Aerolysin (aer) and cytotoxic enterotoxin (alt) were the main virulence factors of A. hydrophila, which could produce toxins capable of causing death. By using two pairs of specific primers for the PCR amplification of aer gene and alt gene, the 280 bp and 250 bp gene fragments were amplified from the isolate, indicating that the isolated A. hydrophila carried virulence factors aerosol and cell toxin. The extracellular enzyme analysis showed that the isolate had amylase, DNA enzyme, urease, and gelatinase activities. The drug susceptibility test by paper extension methods showed that among the test 37 antibiotics, 12 antibiotics including AmmoniaSouth, Florfenicol, and Gentamicin had obvious inhibitory effect on the strain. This paper reported the mass mortality of M. piceus caused by A. hydrophila in Changzhou City, and conducted a detailed study of the pathogen, which would have reference significance for the disease control and health management during M.piceus culture.