During an outbreak of Rift Valley fever (RVF) in livestock near Lake Naivasha, Rift Valley Province, Kenya, 61,347 mosquitoes (1,287 pools) collected in CO2-baited light traps yielded seven viral isolates. Five isolates of RVF virus were recovered from 18,831 Culex zombaensis Theobald and one from 14,439 Mansonia africana (Theobald). One isolate of a Bunyamwera group virus was recovered from 1,175 Aedes quasiunivittatus (Theobald).