Streptococcus agalactiae has been described as one of the most serious bacterial diseases in tilapia culture. This study investigated the isolation and characterization of Streptococcus agalactiae from a condition of mass mortality in farmed Oreochromis niloticus at Baltim region, Kafr El-Sheikh governorate, Egypt during the summer of 2019, and the effectiveness of locally prepared bacterin for prevention of streptococcal disease. Infected fish showed corneal opacity, congestion of spleen, liver, and kidney, the abdominal space filled with watery and bloody ascites, and light-colored nodules on the kidneys, spleen, or liver. Water qualities were measured parallel to the fish sample. S. agalactiae isolates were identified by phenotypic and biochemical VITEK II methods. Ampicillin, Colistin sulfate, would appear as the best antibacterial agents to use in the treatment. The relative level of protection value was 70% in Oreochromis niloticus challenged with a virulent strain of S. agalactiae (107 CFU /1 ml) and vaccinated with locally prepared killed bacterin. Histopathological investigations revealed congested blood vessels in all organ's specimens examined, inflammatory cells infiltration, degenerative changes, different stages of necrosis of the liver, and generalized meningoencephalitis of the brain. The current study recommended that S. agalactia induced many problems in tilapia culture in presence of bad water quality during the summer season, and the vaccination program gives good results and high protection which may be a useful method for the prevention and control of streptococcal infection.