Despite the detection of West Nile (WN) virus in overwintering Culex pipiens L. in New York in February 2000, the mechanism by which this virus persists throughout the winter to initiate infections in vertebrate hosts and vectors the following spring remains unknown. After a blood meal, parous mosquitoes generally do not survive until spring and gonotrophic dissociation occurs in only a small percentage of the population. To investigate vertical transmission as a means of viral survival during interepizootics, we intrathoracically inoculated Cx. pipiens and Aedes albopictus (Skuse) with WN virus and subsequently tested their F₁ progeny for the presence of virus. Among the Cx. pipiens, we recovered virus from two of 1,417 adult progeny that had been reared at 18°C for a minimal filial infection rate (MFIR) of ≈1.4/1,000 and four of 1,873 adult progeny reared at 26°C (MFIR = 2.1/1,000). The mean titer of the positive pools was 10^5.6 plaque-forming units (PFU)/ml (=10^5.9 PFU/mosquito for positive mosquitoes) of virus. Overall, the MFIR was ≈1.8/1,000 for Cx. pipiens. Although reports indicate that Ae. albopictus vertically transmit various viruses in the Japanese encephalitis virus complex, we did not detect WN virus in any of > 13,000 F1 progeny of WN virus-inoculated specimens. Female Cx. pipiens that are vertically infected during the late summer season and then survive the winter could serve as a source of WN virus to initiate an infection cycle the following spring.