Aeromonas hydrophila is a causative agent of motile aeromonas septicaemia (MAS) and has been associated with illness in a broad spectrum of hosts,including mammals,reptiles,amphibians and fish.Aerolysin (aer) gene is one of important virulent factors of A.hydrophila.We have isolated A.hydrophila Ah15 from fish farms,and it has caused high mortality of fish in Hubei Province,China.In order to obtain purified Aer protein for polyclonal antibody preparation,aer gene from A.hydrophila Ah15 was amplified by PCR,subsequently,the aer gene fragment was cloned into T-vector and sequenced.The length of aer gene was 1 482 bp and GenBank number was HQ425625.The deduced open reading frame contained 494 aa in which 24-106 aa was aerolysin/pertussistoxin domain.Based on the aer gene sequences from 23 strains of A.hydrophila isolated from different countries,phylogenetic analysis showed that all strains were divided into three groups and Ah15 fell into the first group which were all isolated from China.A band of 72.2 ku fusion protein was observed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) after expression in Escherichia coli BL21 (DE3) pLysS and highest expression level was detected in the inclusion body 4 hours after induced by adding IPTG.Western-blotting results showed that the polyclonal antiserum obtained by immunizing rabbits with the purified protein could recognize the recombinant fusion protein and the titer was 1∶8 000.Taken together,the recombinant aerolysin has good immunogenicity,and will be a good candidate for vaccine development.