Studies were conducted to determine if two attenuated strains of Rift Valley fever (RVF) virus could be transmitted by Culex pipiens mosquitoes. Both strains (RVF MP 12 and T1) replicated in and were transmitted by female Cx. pipiens after intrathoracic inoculation. Mosquitoes also became infected with and transmitted the RVF MP12 strain after ingesting virus from a blood-soaked cotton pledget. However, because of the low viremias produced in infected animals, it is unlikely that mosquitoes would become infected by feeding on an animal inoculated with either of these viruses. Although both strains were transmitted by mosquitoes after intrathoracic inoculation, there was no evidence of reversion to a virulent virus.