S10 gene sequences of 1 attenuated vaccine strain and 31 Chinese field isolates & 1 South Africa strain of BTV were determined. The results revealed that all 33 S10 gene segments have 822 nucleotides in length with two in-frame initiation codons (nucleotides 20 to 22 and 59 to 61) and a common termination codon (nucleotides 707 to 709), which encodes two proteins (NS3 and NS3A). Nucleotide difference in the sequence of all S10 gene were from zero to 107 bp (86.4% - 100% identity). NS3/NS3A protein showed a light difference from zero to 10 amino acid (95.6% - 100% identity). Phylogenetic analysis of the S10 gene of above strains sequenced and 9 other strains from GenBank, segregated the Chinese viruses into a monophyletic group distinct from US viruses; Nucleotide identity was 85% between China Group and US group. The various Chinese isolates segregated into two phyletic subgroups based on S10 gene sequences. The clustering of viruses was dependent of geographical origin, and independent of host species of isolation, serotype & year of isolation.