The present study revealed that wt RVFV ZH501 stock, which was originally isolated from a patient during the 1977 outbreak of RVFV in Egypt, was made up of two major virus populations with single nucleotide substitution within the Gn gene and that a single nucleotide difference in the Gn gene of wt RVFV substantially affected viral mouse virulence. Furthermore, we observed a remarkably rapid emergence and accumulation of the virulenttype virus in the mice that had been inoculated with a low virulenttype RVFV.