Significant numbers of Culex erythrothorax were collected during arbovirus surveillance in 1987 and 1988 at Las Animas, Bent County, on the high plains of southeastern Colorado. This finding extends the range of this species to the eastern side of the Rocky Mountains. The isolation of western equine encephalitis virus in 1988 from both Cx. tarsalis and Cx. erythrothorax suggests that the latter also may be involved in the endemic cycle of this virus in the area. St. Louis encephalitis, Hart Park, Turlock, and a Bunyamwera group virus were also isolated from the mosquitoes collected.