In August 2011, an enzootic disease characterized by hemorrhage throughout body surface, enlarged spleen and kidney occurred in cage farmed red drum fish, in Dongtou, Zhejiang of China. The diseased fish weighed between 100-300g，and the cumulative mortality within 60 days was higher than 70％. Several bacterial isolates (strainWZMH110819) that exhibited the same phenotypic traits and biochemical characteristics were isolated from the lesions of diseased fish. According to the results obtained from biochemical tests and sequence analysis of the 16S rDNA gene, the bacterial isolates were identified as Streptococcus iniae. In the challenge trials, the LD50 value of the clinical bacterial isolate WZMH110819 was 9.65 ×106 CFU per fish. Moreover, bath exposure or oral administration by Streptococcus iniae also caused a considerable number of deaths in fish. Antibiotic susceptibility tests showed that strain WZMH110819 was sensitive to most of the antibiotics tested including ampicillin, ceftazidime, erythromycin, kanamycin and gentamicin in vitro.These drugs were found to have a strong bactericidal effect (i.e their MIC ≥ MBC) against bacterial isolate (strain WZMH110819). Meanwhile the study on the growth properties of strain WZMH110819 indicated that it grows better in the temperature, pH and salinity range between 20-37℃, 6.0-8.0, and 0-20‰with their optimum values of 30℃, 7.0 and 0‰, respectively. This finding has provided a helpful basis in controlling and prevention of this disease in red drum. It would also provide a theoretical basis in developing a vaccine in order to reduce the negative red drum affects on the species.