Host preferences of four mosquito vectors of arboviruses were studied in Colorado, Texas, and Hawaii to determine the relative potentials of various domestic and wild vertebrates for zooprophylaxis. The source of blood for 1,792 engorged mosquitoes was determined by the capillary-precipitin technique; population counts were made of available vertebrate hosts, and forage ratios were calculated to provide an index of actual host preference. Forage ratios of 242 engorged Culex tarsalis collected from a Colorado farmstead and a city park indicated a high preference for columbiform birds (doves and pigeons) and a lower preference for passerine birds (mostly House Sparrows). Forage ratios for 582 engorged C. tarsalis from three Texas farmsteads where very few columbiform birds were present showed the highest preference for cattle and dogs. Forage ratios for 176 engorged Culex pipiens from a Colorado city park showed a high preference for doves and pigeons. Forage ratios for 709 engorged C. quinquefasciatus collected from a dairy farm and a sylvan habitat on Oahu, Hawaii, indicated a marked preference for dogs, with chickens ranking second. Forage ratios for 83 engorged Aedes albopictus from a sylvan habitat on Oahu, Hawaii, showed an unusually high preference for feral cats, with dogs and man also being preferred hosts. This was the only one of the four mosquito species studied in which human blood was detected. It was concluded that the domestic animals that would have the highest potential values for zooprophylaxis against these four species of mosquito in the types of ecosystems studied would be (listed in the order of host preference): Culex tarsalis—pigeons, cows. and dogs; C. pipiens—pigeons; C. quinquefasciatus—dogs and chickens; and Aedes albopictus—cats and dogs.