occurred in Wenchang,Hainan Province, China in 2014. In this study, the moribund tilapias were collected from different farms for bacteriaisolation. Most of the moribund tilapias displayed typical clinical signs, such as melanosis, corneal opacity,and swimming abnormalities. A total of 19 isolates (TC-1, TC-2, BL1441 WT1459) wereisolated from the liver, kidney, spleen, eye, and brain of the diseased tilapia; then, these isolates were identified bymorphological observation, physiological, and biochemical characteristics, and sequence analysis of the 16S rRNAgene. All of the isolates were identified as –WT1459 strains grew slowly and couldnot utilize trehalose and ribose. In addition, TC-1, TC-2, and BL1441–’WT1459 isolates did not exhibit hemolysis. The geneticcharacteristics of these isolates were analyzed by multilocus sequence typing (MLST), molecular serotyping, andvirulence-related gene sequencing. The results indicated that TC-1, TC-2, and BL1441–S. agalactiae WT1459 isolates were rare typesof Ib-ST261 BL1448 isolates, and the virulence-relatedgenotype of these strains was +. Despite the fact that thenumber of the virulence-related genes of the WT1451 strain was less than that of the BL1441 strain, the formerwas more virulent than the latter in tilapia. In this study, the virulence tests were conducted by intraperitoneal injectionof the BL1441 and WT1451 strains in tilapia. The results of the challenge experiments showed that theWT1451 strain possessed stronger pathogenicity to tilapia than that of BL1441 strain. The WT1451 strain caused85% mortality in tilapia at a dose of 4.5CFU/mL. The BL1441 strain showed weak virulence in tilapia, withcumulative mortality of 65% at a dose of 4.5CFU/mL postchallenge. In summary, the 19 strainsisolated from tilapia in Wenchang were divided into two genotypes based on physiological and biochemical characteristics,16S rRNA gene sequences, hemolysis activity, molecular serotyping, and PCR screening of virulence-related genes. Although reports on the Ib-ST261 strains in tilapia are rare worldwide, thesestrains were highly virulent in tilapia. Therefore, it is very important to obtain data on the epidemiology ofIb-ST261 strains in tilapia in China. To the best of our knowledge, this is the first report on the hypervirulentIb-ST261 strain of , which was isolated from tilapia in China. This study provides aframework for the exploration of epidemiological analysis, vaccine development, prevention, and treatment of