From June through September 2003, we conducted a survey of female Aedes triseriatus (Say) for infection with La Crosse encephalitis virus (family Bunyaviridae, genus Orthobunyavirus, LACV) and West Nile virus (family Flaviviridae, genus Flavivirus, WNV) at three locations in Lorain County, Ohio. To determine infection rate and seasonal variation of both viruses in the Ae. triseriatus population, Ae. triseriatus were collected weekly by using gravid traps and CO2-baited CDC light traps and tested for virus by using reverse transcriptase polymerase chain reaction. In total, 170 pools comprised of 2,143 females were tested for LACV, of which seven were positive; the maximum likelihood estimate of infection rate combined throughout the season was 3.22/1,000. None of 170 pools comprised of 2,158 females tested for WNV were positive. LACV-positive pools were detected between late July and early September.