Frequencies of size classes of deer-mice (Peromyscus maniculatus) femora and humeri from a sample of what are believed to be barn-owl (Tyto alba) pellets collected from Columbia County, southeastern Washington, indicate that most prey were of intermediate age. Behaviors of individuals in this age class indicate that they are more susceptible to predation than older (adult) and younger individuals because intermediate-age individuals are likely to have been in unfamiliar territory during dispersal and establishment of new individual home ranges. Frequencies of size classes of montane-vole (Microtus montanus) femora and humeri in the pellet sample indicate more adults than other age classes were taken as prey. Most individual deer-mice and montane-vole prey were adult or near-adult size when captured.