A mutagenized clone of Rift Valley fever virus (RVFV; MV P12) used in inoculation of 3 pregnant ewes was immunogenic, nonpathogenic, and nonabortogenic. In contrast, inoculation of a matched group of 3 pregnant ewes with parent RVFV induced clinical disease and abortions. Ewes given MV P12 delivered healthy lambs that had RVFV antibody titers of < 1:10 at birth, increasing to .gtoreq. 1:80 after ingstion of colostrum. Ewes inoculated with parent RVFV developed marked viremia, followed by RVFV antibody titers .gtoreq. 1:1,280; ewes inoculated with MV P12 developed low viremia titers and RVFV antibody titers of 1:80 to 1:320. Postpartum challenge exposure of the previously MV P12-inoculated ewes with virulent Zagazig human 501 strain RVFV indicated that the ewes were protected from clinical disease. The RVFV-susceptible female Culex pipiens that fed on the MV P12-inoculated ewes failed to transmit RVFV to hamsters; mosquitoes that fed on the parent RVFV-inoculated ewes became infected and transmitted RVFV to hamsters.