In recent yaars, an epidemic disease with the symptoms of darken appearance, erratic or disorientated swimming behavior, exophthalmia or opaqueness to eyes, inflammation or ulceration of the abdominal skin, and accumulation ascetic fluid in intestines and stomach, etc. outbroken in cultured tilapia in Fujian, Guangdong and Hainan provinces. One strain of bacteria TL60829NA was isolated from the liver of diseased tilapia, the challenged tilapia presented the similar external signs such as exophthalmia and disorientated swimming behavior to those of the natural in fected tilapia, so the isolate was the primary pathogenic agent of the epidemic disease in cultured tilapia. In conventional and Bio Merieux Vitek system tests, isolates were catalase negative, β-haemolytic, Gram-positive cocci and serogroup B. By 16S rDNA amplified and sequenced, the phylogenetic tree of Streptococcicosis showed that the isolate clustered together with Streptococcus agalaciate, the S. iniae strains clustered together, the isolate was identified as S. agalaciate. The isolates were highly sensitive to 28 antibiotics such as ampicillin, kanamycin and penicillin G, and resistant to 15 antibiotics, such as tobracin, cotrimoxazole and ciprofloxacin. Histopathological analysis reveled that the gill's structure was broke down, the epithelial cells swelling; the membrane of the heart ruptured, the cardiac muscle hyperemia, disorderly arranged and leucocytes soaked; the essence structure of liver was destroyed obviously, pellet and fat denaturation; epithelia cells broke away from the intestines; the spleen was hemorrhage, dropsy and cellulose, a large amount of blood source pigmentation dyed pale brown by HE; essential structure of kidney was destroyed, obviously hyperemia and hemorrhage; the choroid and the orbital membrane inflamed and damagee, the crystalline lens fiber broken.