Abstract: ObjectivePathogenic bacteria strains were isolated and identified from diseased tilapia cultured in a fish farm in Santang Town, Nanning City, Guangxi. A drug sensitivity test was conducted to seek out the aetiological agent and provide references for effective prevention and control. MethodThe strains were separated from the brain tissue in dying tilapias using routine method. Pathogenicity of the strains was determined by an artificial infection experiment. The strains were identi-fied using API20Strep and the 16S rRNA sequences. K-B was applied to test the drug sensitivity. ResultFour Gram-positive cocci (GXN01, GXN02,GXN03 and GXN04) possessed strong pathogenicity against healthy tilapia and they might be the aetiological agent causing tilapia to death. The four strains were identified as Streptococcus agalactiae. All the strains shared 99.2%-99.7% similarities with those S. agalactiae strains reported in GenBank (JQ039365,JQ039376,JQ990156, JQ039366, JF423948, HQ645984 and GU217535). These four strains also shared high similarities(99.9%) with each other. Drug susceptibility test results showed that the four strains were sensitive to ofloxacin, cephalosporinVI, cefobid and sarafloxacin hydrochloric,but resistant to gentamicin, norfloxacin, sulfadoxine-6-methoxymethoxypyrimidine and sulfa-methoxy-pyrimidine. ConclusionThe deaths of tilapia might be caused by S. agalactiae infection. Four kinds of drugs (CephalosporinVI, Ofloxacin, Cefobid and Sarafloxacin hydrochloric) could be used for prevention and treatment.