An outbreak of encephalitis due to West Nile (WN) virus occurred in New Yor
k City and the surrounding areas during 1999. Mosquitoes were collected as
part of a comprehensive surveillance program implemented to monitor the out
break. More than 32,000 mosquitoes representing 24 species were tested, and
15 WN virus isolates were obtained. Molecular techniques were used to iden
tify the species represented in the WN virus-positive mosquito pools. Most
isolates were from pools containing Culex pipiens mosquitoes, but several p
ools contained two or more Culex species.