Genetic studies have been carried out on orbiviruses in the Great Island (GI) antigenic subgroup of the Kemerovo serogroup (Orbivrus, Reoviridae) to elucidate the functions of the 10 genomic double-stranded RNA segments. Such studies have shown that segment 4 is the major genetic determinant of neurovirulence (P.A. Nuttall, S.R. Moss, L.D. Jones, and D. Carey, 1989, Virology 172, 428-434), whereas segment 5 of Wexford (WEX) virus and segment 6 of GI virus are the major determinants of serotype specificity (S.R. Moss, C.M. Ayres, and P.A. Nuttall, 1987, Virology 157, 137-144; S.R. Moss, C.M. Ayres, and P.A. Nuttall, 1988, J. Gen. Virol. 69, 2721-2727). In studies with reassortants isolated following dual infection of cell cultures with WEX and GI viruses, the gene combination W4G6 (i.e., viruses deriving segment 4 from WEX virus and segment 6 from GI virus) resulted in nonpathogenic reassortants. Unlike the parental viruses, the avirulent reassortants did not produce clinical evidence of infection in inoculated 2-day-old mice although, suprisingly, they replicated in the brains of the mice. The alternate heterotypic gene combination, G4W5, resulted in typical neurovirulent reassortants. The results indicate that segment 6 of GI virus is able to modulate the phenotypic expression of segment 4 of WEX virus, but not vice versa. Modulation probably results from interactions between the products of these two genomic segments, possibly at the level of virion structure.