Rift Valley fever virus was shown to replicate in Lutzomyia longipalpis after intrathoracic inoculation. Viral titers peaked at approximately 4 days postinoculation [mean titer = 104.0 plaque forming units (PFU)] and remained relatively constant through day 7. A minimum of 6 of 326 sand flies transmitted virus by bite to susceptible hamsters after 5–9 days of extrinsic incubation. Viral titers of sand flies exposed per os declined steadily through day 9. None of 378 flies that had ingested approximately 104.0 PFU of virus transmitted virus when refed on susceptible hamsters.