Thin section electron microscopic examination of several viruses of the Bunyamwera serologic supergroup of arbo viruses confirmed their precise similarities in morphology and morphogenesis and their differences from viruses of other groups. Several viruses that are serologically unrelated to the supergroup were indistinguishable from Bunyamwera virus when observed in the same way. A separate taxonomic group or ‘family’, ultimately to be based upon multiple common physicochemical virion properties, is proposed to encompass the more than 130 viruses listed; the name Bunyaviridae is proposed for this ta