We have characterized the full-length S segment RNA sequences of five human pathogens of the virus family Bunyaviridae, genus Orthobunyavirus. S segment sequences of Fort Sherman, Shokwe and Xingu viruses of the Bunyamwera serogroup, as well as those of Bwamba and Pongola viruses of the Bwamba serogroup, are described. S segment sequences of Bwamba and Pongola viruses represent the first nucleotide sequences characterized for viruses of the Bwamba serogroup. The described molecular and phylogenetic analyses of these and other selected viruses of the genus Orthobunyavirus reveal that a close sequence similarity is shared between the African Bwamba and the predominantly North American and European California serogroups of the genus Orthobunyavirus.