During a 4-year period (1972-1975), mosquitoes were collected, identified, and processed for virus isolation. Laboratory examinations of clinical specimens from suspected encephalitis cases were made to provide etiologic diagnosis. Arbovirus was isolated from 102 pools of mosquitoes (2 LaCrosse, 2 Cache Valley, 10 Flanders, 3 St. Louis encephalitis 83 Trivittatus, and 2 Western equine encephalitis viruses). Laboratory results confirmed recent infections with arbovirus in 44 human cases of diseases of the central nervous system. Chronological and geographical date, information on the mosquito species collected, and the viruses isolated are presented.