Rift Valley fever (RVF) virus infection, dissemination, and transmission rates were determined for Aedes fowleri, Aedes mcintoshi and Culex pipiens 7 or 10 days after sequentially feeding to repletion on RVF virus immune hamsters and RVF viremic hamsters, or after feeding on a mixture of RVF virus immune sheep serum and RVF viremic hamster blood through a pledget. No significant differences in infection or dissemination rates were detected among Ae. fowleri and Cx. pipiens feeding to repletion on immune hamsters before or after feeding to repletion on a viremic hamster. Similarly, no significant differences in infection, dissemination, or transmission rates were observed among Ae. fowleri and Cx. pipiens feeding to repletion on immune hamsters or nonimmune (control) hamsters 0 or 24 hr after inoculation with RVF virus. Infection rates were significantly higher for Ae. fowleri (56/66, 85%) and Cx. pipiens (123/148, 83%) fed only on viremic hamsters than for those interrupted to complete feeding on an immune hamster (Ae. fowleri [24/49, 59%], Cx. pipiens [66/131, 50%]) or a nonimmune hamster (Ae. fowleri [32/51, 63%], Cx. pipiens [69/127, 54%]). However, no significant differences were detected in infection, dissemination, or transmission rates among Ae. fowleri, Ae. mcintoshi or Cx. pipiens fed on a viremic hamster and interrupted to complete feeding on an immune vs. a nonimmune hamster. Results from interrupted feeding experiments were significantly different from pledget feeding experiments. Infection rates were significantly lower for both Ae. fowleri and Cx. pipiens ingesting a mixture of viremic hamster blood and RVF virus immune sheep serum through a pledget than for those ingesting viremic hamster blood or a mixture of viremic hamster blood and nonimmune sheep serum through a pledget. Results from experiments in which mosquitoes fed directly on hamsters indicate that Ae. fowleri, Ae. mcintoshi and Cx. pipiens are efficient vectors of RVF virus whether or not they also feed on immune or nonimmune vertebrate hosts.