The threshold of infection with Wesselsbron virus was lower for Aedes aegypti than for Culex quinquefasciatus, but after ingesting higher doses of virus, C. quinquefasciatus more frequently became infected. Nevertheless, once infection was initiated in A. aegypti, the virus replicated to a high titer in a greater proportion of mosquitoes. The net result was that A. aegypti, after ingesting small amounts of virus, transmitted virus more efficiently and after a shorter extrinsic incubation period than C. quinquefasciatus; after the ingestion of high doses, both species transmitted infection with equal efficiency.