During an arbovirus surveillance in New York State conducted from 1972 to 1977 a total of 918,047 wild-caught mosquitoes, approximately 60% Aedes species, were examined for virus in 20,616 pools. Five genera yielded 228 isolates: Eastern equine encephalomyelitis (EEE), 39; Highland J strain of Western equine encephalomyelitis (WEE), 7; California encephalitis complex (CAL), 73; Cache Valley (CV), 8; and Flanders (FLA), 96; there were also 5 unidentified virus strains. Both EEE and WEE were isolated primarily from Culiseta melanura, CV from Aedes and Anopheles, FLA from Culex pipiens, and CAL viruses from Aedine mosquitoes. The CAL and FLA viruses had the widest geographic distribution, whereas EEE, WEE and CV isolates were limited to central and/or southeastern New York. This surveillance, supplemented by virus isolation attempts and serological studies of domestic and wild vertebrates, identified the endemic and epidemic occurrence of several human pathogenic arboviruses and indicated potential vector species for CAL and EEE viruses, which are of the greatest public health importance among arboviruses known to occur in New York State.