Field studies were conducted during an epizootic of vesicular stomatitis virus (VSV) in Colorado to further assess the possible role of insects in the transmission of VSV. Insects associated with domestic livestock were collected at 11 premises along the Front Range of the Rocky Mountains in Colorado during the 1982 epizootic of vesicular stomatitis. Insects were pooled by date, location, species, and sex and were processed for virus isolation in three cell culture systems. Thirty-four isolates of vesicular stomatitis virus, New Jersey serotype, were obtained from 51,036 insects. Of these, 27 isolates were from Musca domestica (126 pools/5,285 specimens), 5 from other nonhematophagous Diptera (56 pools/936 specimens), and 2 from unengorged black flies (Simuliidae) (55 pools/1,221 specimens). Results suggest that nonblood-feeding insects, such as houseflies, play a role in VSV transmission and that black flies also serve as vectors. KEY WORDS Insecta, VSV, VSV from nonhematophagous insects, VSV from Simuliidae VESICULAR STOMATITIS (VS) is a disease of cattle, horses, swine, and various wild vertebrates; it is caused by several viruses of the family Rhabdoviridae, genus Vesiculovirus, found in North and South America and Asia (Yuill 1981). VS is enzootic in the tropical and subtropical regions of the Americas, but it can spread northward into the United States and Canada during the summer months, appearing first in states along the Gulf of Mexico in April or May and later as far north as Manitoba, Canada. The natural history of VS virus in North America, including endemic maintenance and epizootic transmission, remains uncertain despite many years of study. Outbreaks often have appeared more or less simultaneously within a broad geographical area. The seasonal occurrence of disease and the spotty geographic distribution of cases suggest insect transmission. VS Indiana (VSI) virus has been isolated frequently from sand flies (Diptera: Psychodidae) caught in the wild in tropical America (Yuill 1981) and from mosquitoes (Diptera: Culicidae) collected in New Mexico (Sudia et al. 1967). Replication of the virus and transmission by bite have been demonstrated in sand flies (Tesh et al. 1971), and high rates of vertical (transovarial) transmission also have been found (Tesh et al. 1972). The New Jersey serotype (VSNJ) has been isolated from black flies (Diptera: Simuliidae) in Colombia (Theiler & Downs 1973) and from mosquitoes collected in Ecuador (Calisher et al. 1983) and Guatemala (C.H.C., unpublished data). Dur