Edwardsiella tarda is a gram-negative pathogen with a wide host range,which can infect many kinds of fish and lead to extensive losses in aquaculture.The pathogenesis of E.tarda is not clearly understood.So far as we know its virulence factors include hemolysins,chondroitinases,dermatonecrotic exotoxic substances,catalases,etc. The latest study indicates that the type three secretion system(TTSS) ofE.tarda is tightly associated with bacterial pathogenesis. The gene structure of the TTSS has been elucidated and relational research is on the way.