The meningitis was the characteristic clinical manifestation in Nile tilapia(Oreochromis niloticus) infected by Streptococcus agalactiae. To know the degree and extent of inflammation as well as the presence of necrosis in the brain,we build an intraperitoneal infection model and develop a new clinical and histopathological scoring system for S.agalactiae in Nile tilapia.43 healthy nile tilapias,were randomly divided into 4 groups:control group(n=10),low dose group:1.0×10⁶cfu group(n=11),middle dose group:1.0×10⁷cfu group(n=11),high dose group:1.0×10⁸cfu group(n=11).Clinical symptoms and cerebral histopathological changes(cerebrum,optical lobe,cerebellum and medulla oblongata included)was oberserved in our study,we scored these three groups and obtained the best one.The infected nile tilapia died in second day post infection,however,neurological symptoms were not apparently observed in the dead fish.Survivors developed constant aberrant swimming,exophthalmia and corneal opacity.According to microscopical examination of four regions of the brain,large amount of bacteria stained bluish violet colonized the subarachnoid space and cerebral pia mater.Histopathology revealed the changes of leptomeningitis and meningoencephalitis,including congestion/hemorrhage,edema,loose and thickening in meninges,inflammatory cell neutrophilic leukocytes infiltration,superficial periplast of brain inflammatory cell infiltration and lysis spongy-structure,gliocyte hyperplasiain,and so on.Compared with the scores,we found that the group of 1.0×10⁷cfu was the best dosage,which showed significant clinical symptoms,the minimum diversity value and the condition of the sick fish contributed to the observational study.We chose the dose of 1.0×10⁷cfu to inject nile tilapia intraperitonealy to develop a model of S.agalactiae meningitis with the 100% success rate on the third day post infection.