The Water Resources Branch, Center for Disease Control (CDC), assessed mosquito production in the area surrounding Lewis and Clark Lake, on the Missouri River in South Dakota and Nebraska, during the period from June through August, 1975. Extensive collections of mosquito larvae were made. Eleven of 360 collections from Nebraska contained mosquito larvae infected with Coelomomyces psorophorae, whereas none of 135 collections from South Dakota contained infected larvae. This is the first report of Coelomomyces infecting mosquitoes in Nebraska. Only 4th instar larvae of Aedes vexans were found infected. Fourth instar larvae of 9 other mosquito species were present in some of the positive collections, but none were infected. Infection rates in Ae. vexans from the 360 Nebraska collections, and 5.7% of the 1,451 4th instar Ae. vexans from the 11 collections were positive for C. psorophorae. A. copepod, Cyclops navus, was present in some collections, but its possible role as an obligate alternate host for C. psorophorae was not determined.