A previous investigation showed that MX10 virus, recently isolated in China, belongs to the Oriental-Australian (O/A) genotype of Sindbis virus (SINV) (Wang Jinglin, 2011, ATMH). Similar to the MRE16 isolate, the prototype O/A genotype of SINV, two derivate viruses with obviously different plaque morphologies were derived from MX10 virus, which were accordingly denoted as MX10-LP and MX10-SP. MX10-LP virus exhibited higher neurovirulence in neonatal mice than MX10-SP virus. Analysis of the complete genome revealed seven nucleotide differences between MX10-LP and MX10-SP. Compared with MRE16 virus, MRE16SP virus has a deletion of 30 aa in the E2 gene (200-229), which has been shown to be the molecular basis for the different plaque morphology. However, the MX10-SP virus did not have the 30-amino-acid deletion in the E2 gene. These results demonstrate that the molecular basis for the different plaque morphology of MX10 virus, the first strain of the O/A genotype of SINV isolated from China, is different from that of the prototype MRE16 virus.