The flaviviruses consist of about 70 viruses that include some important pathogens that are responsible for a number of serious diseases, such as yellow fever, dengue fever, and various encephalitides (Porterfield, 1980; Shope, 1980). They are transmitted to humans by arthropod vectors, i.e., mosquitoes and ticks (Chamberlain, 1980) and are also called arboviruses. The number of known flaviviruses will undoubtedly increase in the future as more viruses are isolated from various hosts and their vectors. Until recently, the flavivirus genus was included with three others, alphavirus, pestivirus, and rubivirus, as part of the Togaviridae family (Porterfield et al., 1978). With the accumulation of experimental data, it has become increasingly clear that flaviviruses have substantially different genomic organization and mechanisms of replication and gene expression from those of alphaviruses, the other major genus of the Togaviridae family. Therefore, the possibility that the two genera have diverged from the same ancestor is remote. Consequently, flaviviruses are now classified in their own family, Flaviviridae (Westaway et al., 1985).