Abstract: Four main extracellular virulence factors of Aeromonas hydrophila， including aerolysin（aer） ， hemolysin（hlyA） ， extracellular protease（epa）and cytotoxic enterotoxin（act） ， were investigated in 60 isolates of A. hydrophila by PCR. These 60 stains were isolated from diseased fishes from Jiangxi and Guangdong province in 2008-2009. The majority（58.33%）of the strains carried the four virulence genes. aer + hlyA + epa + act + virulence genotype isolates were major in A. hydrophila isolates . 12 representative strains from these genotypes were randomly selected， and individually injected into swordtail. The results showed that the isolates with more virulence factors had stronger pathogenicity to swordtail. aer + hlyA + epa + act + genotype isolates were more virulent to the swordtail compared to others.