During August and September of 1991, an epizootic of eastern equine encephalitis (EEE) virus in horses occurred in Wayne and Holmes countries, OH. This was the first recorded epizootic of EEE virus in the state. Twelve horses were confirmed positive for EEE virus through virus isolation or seroconversion, and seven additional horses with compatible symptoms were in close spatial and temporal proximity to the confirmed cases and were presumed to have died from EEE virus. The outbreak was centered around the Killbuck Wildlife Area, a 2,147-ha tract maintained by the state, half of which consists of wooded swamp and marsh. Mosquitoes were collected in upland areas before the epizootic and in the swamp basin at the end of the epizootic to identify the mosquito species involved in EEE virus transmission. We collected and tested 22,095 specimens for the presence of virus. EEE virus was isolated from one pool of the most likely epizootic vector, Coquillettidia perturbans (Walker). The minimum infection rate for EEE virus in this species was 0.1/1,000. Dense populations of Aedes vexans (Meigen) and Culex salinarius Coquillett occurred in the area, but their densities peaked after the epizootic. It is unlikely that these species were involved in epizootic transmission. IgM antibody to EEE virus was detected in three bird species collected in the swamp.