We conducted a study to determine why Dermacentor andersoni switch hosts as they develop, and in so doing we tested and accepted the assumption that this tick prefers to feed on hosts from which it is collected most often. Adult, nymphal, and larval ticks were placed in capsules on mice or guinea pigs. Adult ticks attached more often and imbibed more blood from the larger guinea pigs than from smaller mice. Feeding by two adult female ticks caused 5 of 10 mouse hosts to die. Nymphs obtained more blood from mice than guinea pigs, but attachment was similar for both hosts. Larvae also imbibed more blood from mice than from guinea pigs, in addition to exhibiting a preference for mice as shown by more frequent attachment on that host than on guinea pigs. Results support epidemiological explanations for the maintenance of Colorado tick fever and Rocky Moutain spotted fever, two important diseases transmitted by D. andersoni.