The high rate of Keystone virus recovery from Aedes atlanticus collected in 1971 and 1972 suggested that these mosquitoes might be emerging infected. To demonstrate the suspected transovarial transmission of this virus, developmental stages of A. atlanticus were collected from the field. Larvae were identified and pooled for virus isolation in suckling mice. Pupae were allowed to emerge in the laboratory, then were segregated by sex and pooled for virus isolation. Keystone virus was recovered from larvae, reared males and reared females, demonstrating transovarial transmission of the Keystone strain of California encephalitis by A. atlanticus.