A mosquito surveillance program designed to prevent future human cases of Western equine encephalitis or Saint Louis encephalitis was established in 1983 on Indian reservations under the jurisdiction of the Aberdeen Area Indian Health Service in Iowa, Nebraska and South Dakota. Standard New Jersey light traps were operated on all reservations throughout the summer season while CDC miniature and encephalitis virus surveillance traps were employed for shorter periods of time on 4 reservations. Aedes vexans was the most abundant species except on the Pine Ridge, Rosebud, Winnebago and Yankton Sioux reservations in which Culex tarsalis predominated and for the Sac-n-Fox where Aedes trivittatus occurred with the greatest frequency.