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A virus surveillance project was established and maintained during 1972 along 10 major river drainages in six states. Mosquitoes, biting flies, and blood specimens from sentinel equines were collected during 83 field trip visits to 141 arthropod collecting sites and 22 sentinel locations from April into December 1972. There were 173,074 mosquitoes tested and 303 arboviruses isolated from 11 of 41 species. From 13,388 biting flies tested, 8 arbovirus isolations were obtained in 1 of 5 species. There was no isolation of Venezuelan equine encephalitis (VEE) virus. Western equine encephalitis (WEE) virus isolates were the most numerous and were followed by Turlock, St. Louis encephalitis, Hart Park, California encephalitis, and Bunyamwera (BUN) group viruses. The first isolation of WEE from the mosquito Culex (Mel.) erraticus is reported, as is the extension of the ranges for Buttonwillow virus from California to New Mexico and Texas. Also a single isolation of the BUN group from Culicoides variipennis extends the range of this virus-vector relationship from California to Texas. New distribution records for mosquito species previously unreported for Arizona, Louisiana, New Mexico, and Oklahoma are reported. The sentinel burros detected WEE serologic conversions at two sites in New Mexico and at one in Texas. The surveillance project provided state and federal officials with current information on the status of arbovirus activity, including the absence of VEE activity during 1972, and it demonstrated the existence of the potential for WEE epizootics and epidemics throughout a wide geographic area of the Western United States.