Ecological investigations in Lunyo Forest, Uganda during 1968 yielded isolations of Germiston and Witwatersrand viruses from mosquitoes and rodents. Witwatersrand virus was shown to be ether and sodium deoxycholate sensitive and to contain RNA. Virion size was estimated to be approximately 95 m. The virus was pathogenic and grew to high titre in suckling and weanling mice, golden hamsters, and two cell culture systems (BHK-21 and VERO). Electron microscopy revealed viral particles resembling Bunyamwera supergroup viruses and a number of ungrouped tick-borne arboviruses.