A surveillance program for mosquito-borne arboviruses was conducted in Connecticut following an epizootic of eastern equine encephalitis (EEE) in horses and domestic birds during 1990. Mosquito trapping was done weekly using CO2-baited miniature light traps at 12 freshwater swamp sites that were located mostly in the southeastern portion of the state. Trapping was conducted from June 27 to October 11, 1991 and from June 2 to September 30, 1992. Totals of 7,435 (1991) and 13,912 (1992) adult female mosquitoes representing 21 species in 7 genera were collected and assayed for arboviruses. Virus isolates were identified by ELISA using reference antibody of California encephalitis, EEE, Highlands J (HJ), Jamestown Canyon (JC), LaCrosse, and St. Louis encephalitis viruses. Culiseta melanura was the most common species trapped each year, followed by Aedes canadensis, Aedes cinereus, and Coquillettidia perturbans. The most abundant univoltine snowmelt species was Aedes abserratus. Three isolates positive for JC virus were obtained from Ae. abserratus, Ae. canadensis (new state record), and Ae. cinereus (new state record) that were collected from 2 different sites in June (1992) and July (1991 and 1992). Six isolates positive for HJ virus were made from Cs. melanura and one isolate from Ae. cinereus (new host record) collected in mid- to late September, 1992 from 3 locations. Based on repeated virus isolations in this and other studies, high field infection rates, and its relative abundance, Ae. abserratus appears to be a principal vector of JC in Connecticut. However, the prevalence and importance of JC as a human disease in the state are unknown.(ABSTRACT TRUNCATED AT 250 WORDS)