The cDNA of RBSDV S9 was amplified by RT-PCR using the total RNA of infected maize leaves as the template, and then the PCR product was cloned into pBluescriptII SK. After the DNA sequence was determined, the S9-1 gene was subcloned into E. coli expression vector pET-21d. SDS-PAGE analysis revealed that a 40 kD protein was highly expressed. N-terminal sequence analysis confirmed the 40 kD protein was S9-1 protein. Antiserum was prepared using the purified protein. Western blotting indicated that S9-1 protein could be detected in the infected maize leaves.