Stable traps, each baited with a jackrabbit and either a chicken or a pheasant, collected more than 21,000 mosquitoes in the Sacramento Valley, California, in 1972 and 1973. The focus of interest was the feeding behavior of Culex tarsalis, a primary vector of encephalitis viruses. Generally, feeding success was less and feeding rates on the jackrabbit were greater when larger numbers of mosquitoes were collected, or when a bird was exposed that was less receptive to mosquito feeding. Greater feeding rates on the jackrabbit apparently resulted from decreased feeding on the avian host and from diversion of mosquitoes to the jackrabbit. Figures are given that show how changes in feeding rates on birds affect the probabilities that a mosquito that feeds twice would feed both times on a bird, or on a bird and then on a mammal. Relatively small collections of nine mosquito species other than C. tarsalis revealed distinct species differences in feeding behavior. Aedes melanimon showed a preference for jackrabbit, which is relevant to its role as a vector of western equine encephalitis and California encephalitis viruses.