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resh water environments. In freshwater environments, tilapia is most affected by Streptococcus agalactiae, but has limited information of 5. agalactiae for tilapia culture in brackish. Therefore, this study aims to detemiine tbe effects and characteristics of S agalactiae on tilapia cultured in brackish water. Identified of S. agalactiaewas screened by biochemical method and confirmed by PCR with 16S-23S rRNA Additionad, the effect of temperature, pH and salinity on S. agalactiae growth in in-vitro condition were applied in this study. The results showed that: the optimum conditions for S. agalactiae grow were temperatures at 35''C, pH = 7 and salinity at 10%o. 5. agalactiae was causative agent of tilapia reared in brackish water with such clinical signs as: hemorrhage origin of fin, opaque eye, broken eye, empty stomach, swollen liver and abdomen containing fluids. Tbe biochemical and molecular biology methods have the same results for identifying species of S. agalactiae and Keywords: Tilapia, Stieptococcus ag