Abstract: The families of GIFT strain Nile tilapia (Oreochromis niloticus) were established by pair-mating and the juveniles (50~60 g) in each family were challenged by Streptococcus agalactiae (GD001 strain). According to the median lethal concentration (LC₅₀) and infection mortality rate of each family, we evaluated the disease resistance of GIFT strain Nile tilapia to the challenge of S.agalactiae GD001 strain. The results show that: 1) 67 full-sibling families were successfully established, and 53 families (79.1%) were maintained based on their reproductive performance; 2) The LC₅₀ of S.agalactiae GD001 strain to tilapia was 4×10⁸ cfu·mL^-1; 3) Among the 53 families, 11 families had survival rate of over 90%, 15 families (70%~89%), 19 families (30%~69%) and 8 families (less than 30%). It is suggested that the disease resistances of different GIFT families challenged with S.agalactiae were significantly different (P < 0.05).