Abstract: Systemic investigations on the pathogen of epidemic septicemia,a new infectious disease in fishes(mainly silver carps,tilapia)which occurred among cultured fishes in South China in summer to early-autumn of 2003 were carried out.Two strains of Edwardstella tarda(Et)and a strain of Citrobacter freundn(Cf)were isolated from kidney of moribund fishes with hemorrhage in dermis,vent,fin,gill and internal organs.Four-tube method and clinic symptom indicated that two isolates belong to Et and a isolate Cf.In addition.VITEK-JR30 system was used to identify these isolates and reconfirmed the results.Pathogenic test documented that the bacteria are the causative agents of the disease.After experimentally infected by the bacteria via intraperitoneal inoculation,fishes died of hemorrhagic scpticemia,similar to naturally infected fish.The piscine diseases were effectively controlled by using sensitive drugs which had been proven to be strongly susceptive to these strains.