Technical Abstract: Female mosquitoes commonly exhibit patterns of blood feeding from vertebrate hosts, a behavior that strongly influences mosquito pathogen infection and transmission. The vertebrate host dynamics of the mosquito transmitted arbovirus, West Nile virus (family Flaviviridae, genus Flavivirus, WNV) in sagebrush ecosystems remain unknown. Cytochrome b (cyt b) gene sequence analysis was used to identify potential sources of WNV in sagebrush ecosystems as part of a study to investigate impacts of the disease on greater sage-grouse (Centrocercus urophasianus), an ecologically imperiled species restricted to sagebrush habitats in western North America. Two hundred and forty-four female mosquitoes captured in light traps and with visually-detectable blood in the gut were analyzed; of these 197 mosquitoes provided 207 DNA sequences for determination from their host blood meals. Of 84 engorged Culex tarsalis analyzed, 60 blood meals were mammalian and 24 were avian in origin, representing 9 mammalian and 14 avian species. No avian species were judged to be primary hosts, but among mammals both mule deer (28%) and cattle (25%) were primary mammalian hosts for Cx. tarsalis. The primary mammalian host for Aedes vexans was white-tailed deer (40%), while the primary avian host was ring-necked pheasant (Phasianus colcichus L.) in 90 % of the avian blood meals. For Aedes dorsalis, no avian blood meals were detected and pronghorn antelope (43%) and cattle (37%) were primary mammalian hosts. A measure of diversity derived from Shannon index values, indicated that Cx. tarsalis, Ae. vexans, and Ae. dorsalis differed in terms of the host species from which blood meals were taken. Furthermore, a comparison of three mosquito sampling areas that represented differing physiographic features showed that Cx. tarsalis adjusted feeding preferences based on host availability. These findings complement other entomological and virological studies in providing useful information concerning the role of mosquitoes in transmitting WNV among wildlife in the sagebrush steppe community.