Aedes japonicus japonicus was introduced into the northeastern USA in 1998 and has since spread to more than 25 states. Because this species has been shown to be a competent laboratory vector of several viruses, readily feeds on large mammals, and has become a pest in several areas, there is concern that it might serve as a vector of Rift Valley fever virus (RVFV) should that virus be introduced into North America. Infection with RVFV causes mortality in > 90% of young domestic ungulates (e.g., calves, kids, and lambs), as well as causing a febrile illness and occasional deaths in humans. Therefore, we evaluated Ae. j. japonicus captured in North Carolina and in Maryland for their ability to serve as potential vectors for RVFV. After feeding on infected adult hamsters, these mosquitoes were tested for infection, dissemination, and the ability to transmit RVFV after incubation at 26 degrees C for 7-28 days. Both the Maryland and North Carolina populations of Ae. j. japonicus were highly efficient laboratory vectors of RVFV, with infection rates > 90% and dissemination rates > 84% for those mosquitoes that fed on hamsters with viremias > or = 10(8.5) plaque-forming units/ml. Thus, Ae. j. japonicus should be targeted for immediate control should RVFV be introduced into an area where this mosquito is now present.