Abstract: In recent years,red drum(Sciaenops ocellatus) became the economic important species of marine cage culture in Zhejiang Province. During September to October 2001,continuous mortality of cagecultured red drum was recorded in Zhoushan,with the symptoms of skin lesions,exophthalmia and disorientation,etc. Two bacterial strains SO22 and SO23 were isolated fromthe kidney of moribund red drum. The pathogenicity of these two isolates to red drum,tilapia and mouse was tested. The LD50ofSO22 and SO23 to red drum were 418 × 108CFU/fish and 119 × 107CFU/fish,to tilapia 218 × 108CFU/fish and 813×107CFU/fish,to mouse 916×106CFU/ mouse and 412 × 106CFU/ mouse,respectively. The challenged fish presented the similar externalsigns to natural infected red drum,such as exophthalmia and disorientation. These two isolates were preliminarily identified asStreptococcus iniae with the characteristics of gram positive cocci in chains, catalase negative and beta2hemolytic,growth under10 ℃and non growth up 45 ℃,hydrolyzation of esculin and arginine,Voges Proskauer,urease,and hippurate tests negative,fer mentation of glucose,salicin,sucrose and starch,non fermentation of arabinose,inulin,lactose,melibiose,raffinose and sorbitol.They were highly sensitive to ampicilin,vancomycin and cefazolin,while resistant to gentamicin,compound sulfamethoxazole,lincomycin and norfloxacin. Polymerase chain reaction(PCR) of 16SrRNA gene was carried out by using the primers specific forStreptococcus iniae and a 300bp product was identified with 2% agar electrophoresis,which confirmed the identification of thesetwo isolates as Streptococcus iniae.