In recent years, a number of flaviviruses that replicate only in an arthropod host have been discovered and characterized. We describe here the isolation and characterization of a new mosquito-only flavivirus in this group. The virus was isolated from Culex tritaeniorhyncus mosquitoes collected in Vietnam in 2002 and was found to be genetically different from mosquito flaviviruses described previously. We propose the isolate be named Quang Binh virus.