After the 1999 West Nile (WN) encephalitis outbreak in New York, 2,300 over
wintering adult mosquitoes were tested for WN virus by cell culture and rev
erse transcriptase-polymerase chain reaction. WN viral RNA and live virus w
ere found in pools of Culex mosquitoes. Persistence in overwintering Cx pip
iens may be important in the maintenance of WN virus in the northeastern Un
ited States.