To investigate the functions of the capsular polysaccharide synthetic genes (GBS) isolated from Nile tilapia (cpsE and , were constructed by homologous recombination. Genomic DNA of GBS was used as a template to amplify the up and down homologous fragments of , whereas the pSET1 plasmid was used as a template to amplify the chromosomal chloramphenicol resistance gene ( and pSET4s-?/SUP> polymerase chain reaction (PCR) method. The recombinant plasmids pSET4s- were transformed into wild-type GBS by electroporation. Double-crossover and plasmid loss strains were obtained by changing the culture temperature. Finally, were screened for chloramphenicol resistance and the mutations were confirmed by PCR, real-time PCR, and DNA sequencing. To characterize , their growth rate, capsule thickness, capsular sialic acid content, and virulence were compared with those of wild-type GBS. The results showed that the growth rates of the wild-type, strains did not significantly differ. However, the capsule thickness, capsular sialic acid content, and virulence of were significantly lower than those of the wild-type strain. Further research suggested that is the critical synthetic gene of the capsular polysaccharide of GBS, whereas is important for capsular polysaccharide sialylation. The deletion of not only significantly reduced the capsular sialic acid content of GBS isolated from fish, but also significantly impaired its virulence.