Pigeons were exposed over several years in the field in 2 ecologically distinct regions in the inland plateau region of South Africa, viz, moist Highveld and arid Karoo, and infections by Sindbis and West Nile viruses were recorded in these pigeons by periodic tests for viral antibodies. In both regions, infections by both viruses occurred during most years. The monthly incidence fluctuated and was usually low. Infections were recorded in the Highveld from December to May and in the Karoo from January to June inclusive, i.e., from the middle of the warm, wet season through the early part of the cold, dry season. During the remainder of the year, no infections were recorded. Infections by both viruses occurred at a higher overall rate in the Karoo. West Nile virus caused a higher overall rate of infection than Sindbis virus in both regions. Culex univittatus, the primary vector of both viruses on the plateau, was trapped in significant numbers inside the pigeon cages at both study areas, and its presence in these cages over a l2-month period at the Highveld locality correlated with the seasonal prevalence of pigeon infections.