From mid-December 1983 until mid-April 1984, there was an epidemic of Sindbis (SIN) virus infection in the Witwatersrand-Pretoria region in which hundreds of human cases were diagnosed clinically. Twenty-eight of these diagnoses were confirmed in the laboratory by seroconversion as being infections with SIN virus, and 5 cases of infection with West Nile (WN) virus were also found. Attempts to isolate virus from 66 patients, mainly from serum specimens, were unsuccessful. Infection rates for the mosquito vector Culex univittatus, collected at localities on the Witwatersrand in February and March, were mostly higher for both SIN and WN viruses than in previous years. The highest rates determined were 5.4 (SIN) and 9.6 (WN) per 1 000 mosquitoes. It is concluded that an epizootic of both viruses occurred which was manifested by a high level of viral activity in the feral Cx. univittatus-bird transmission cycle. Cx. univittatus efficiently transferred infection of SIN virus from this cycle to man to cause the epidemic of infection with that virus but it is unclear why there were apparently only a few cases of WN virus infection.