The dominant bacterial strain,designated as LFY-08-23,was isolated from the diseased hybrid tilapia，Oreochromis niloticus×Oreochromis aureus in Hainan Province.The biochemical and physiological characteristics of the isolates were studied by using conventional method and Biolog System.The test showed that the isolates were Gram-positive cocci arranging in chains,catalase negative,β-haemolytic on 5% sheep blood agar and serogroup B.According to their 16S rRNA gene sequence and molecular phylogenetic dendrogram,the strain LFY-08-23 was most close to Streptococcus agalactiae with a similarity of 99.8%.The virulent test was carried out by using the pure culture of the bacteria artificially infected Ictalurus punctatus and Carassius auratus gibelio.The two species of experimental fish all died at bacterial concentration of 1.0×109 cfu/mL.The morphological and biochemical characteristics of the re-isolated bacteria from the artificially infected diseased fish were the same as those of the original infected bacteria.The susceptibility test to antibiotics demonstrated that the isolate strain LFY-08-23 was sensitive to most of the tested antibiotics,especially highly sensitive to 13 kinds of antibiotics,such as Cefotaxime,Ciprofloracin,Rifampici，and resistant to 5 antibiotics,such as Kanamycin,Streptomycin,Gantrisin.