is a major bacterial pathogen that has caused severe economic losses in many species of freshwater, marine and estuarine fish worldwide. ScpB is a highly conserved surface protein among all group B streptococcus (GBS) strains and is an attractive surface-exposed antigen for inclusion among vaccine components against GBS. In this study, the strain isolated from diseased tilapia farmed in Guangdong province, China. The gene contains a 2 799 bp open reading frame (ORF), encoding 932 amino acids. The molecular mass of the deduced protein was 103 kD. Blast analysis showed that it shares high similarity with sequences of human GBSs registered in GenBank. Prokaryotic expression vector pET32a (+) was used to construct a recombinant expression vector pET32a (+)- BL21 (DE3). Colonies containing the recombinant expression vector pET32a (+)-were selected and then induced to express using 0.5 mmol/L IPTG for 8 h at 37. The expressed recombinant protein was purified by nickel chelate affinity chromatography and ultrafiltration tube enrichment. SDS-PAGE analysis and western blotting showed a specific protein band of about 121 kD. The recombinant strain could produce large amounts of ScpB protein, mainly in the form of an inclusion body. To analyze the immunogenicity of the recombinant protein, the purified fusion protein and Freund’s adjuvant were mixed according to a certain proportions to produce vaccines. Three immune dosages, 1μg/g(F1), 3 μg/g(F3) and 5 μg/g(F5) were used. Four weeks after immunization, tilapia were challenged by artificial infection with the GBS ZP-N strain, which was previously isolated and confirmed to be a tilapia pathogen by our laboratory. The recorded relative percent survivals (RPS) of the vaccinated groups ranged from 69.99% to 89%, of which group F5 has the highest RPS. The lysozyme activities, superoxide dismutase (SOD) activities and antibody levels (OD_450nm) were tested weekly until the end of the experiment. The results showed that after immunization, there were significantly higher lysozyme activities, superoxide dismutase (SOD) activities and antibody activities(OD₄₅₀) in the vaccinated fish compared with the control group (<0.01). The results of this study showed that recombinant protein ScpB has a strong immunogenicity and protective effect, laying the foundation for further study of polypeptide vaccines for