A low volume formulation of propoxur wettable powder suspended in larvicidal oil was evaluated on 4 occasions in Kern Co., California during 1983 using fixed and rotary wing aerial application systems. Culex tarsalis abundance was suppressed significantly by all sprays, although reduction below the virus maintenance threshold of 30 females per trap night was achieved only at one foothill site. Western equine encephalomyelitis virus minimum infection rates decreased significantly after serial applications on 2 occasions at a valley site. However, virus persisted in the spray zone at minimum infection rates of greater than 1 per 1000 Cx. tarsalis females tested and transmission of virus to sentinel chickens continued. The parity rates were reduced significantly at 2 semi-isolated foothill sites, but not at a valley site where elevated autogeny rates increased the reproductive age of the host-seeking population. Spraying during late afternoon by helicopter resulted in better control than early morning applications by fixed wing aircraft at a valley site.