Four species of Culex were tested for viral susceptibility by feeding on chicks and pigeons circulating different levels of either West Nile (WN) or Sindbis (SIN) viruses. After 14-19 days mosquitoes were tested individually for virus. The following concentrations of virus were found to infect 10% of the mosquitoes: C. pipiens Linnaeus-1.0 logs (WN) and 6.4 logs (SIN) C. quinquefasciatus Say=fatigans Wiedemann-1.4 logs (WN) and ca. 5.0 logs (SIN) C. univittatus Theobald-<1.0 logs (WN) and 2.5-3.2 logs (SIN) and C. theileri Theobald <2.4 logs (WN) and 2.5-3.2 logs (SIN) C. pipiens was also fed on suspensions of virus in defibrinated blood which gave a much higher 10% infection threshold for WN virus (ca. 5.2 logs), although the threshold for SIN virus (>5.0 logs) remained essentially the same. Similarly, C. quinquefasciatus=fatigans had previously been found rather refractory to WN virus when infected in this manner, although there was no difference with SIN virus. Hence, caution is indicated when using blood-virus mixtures for infecting mosquitoes if information on their natural susceptibility is sought. The vector potential of C. pipiens and C. quinquefasciatus=fatigans with WN virus on the South African Highveld is assessed in the light of these findings. The 10 percent threshold levels of infection previously determined for C. univittaus and C. theileri and both viruses using blood-virus mixtures were comparable to the thresholds determined when viremic birds were used to infect these mosquitoes.