Eight species of small mammals were evaluated as potential hosts for American dog ticks, Dermacentor variabilis (Say), in an upland, tallgrass prairie study site in central Oklahoma. Only hispid cotton rats, Sigmodon hispidus, and deer mice, Peromyscus maniculatus, were found to be important hosts for immature D. variabilis. Although D. varibilis larvae and nymphs frequently infested both cotton rats and deer mice, cotton rats were the most important host species for both immature stages in the study area. Cotton rats constituted 63.2% of the total 530 small mammals captured and were hosts to 85.2% of all larvae and 88.7% of all nymphs. Deer mice accounted for 19.8% of all small mammals captured and were hosts for 14.5% of the larvae and 10.8% of the nymphs recovered. The remaining small mammal species were hosts for less than 1% of the immature ticks collected. Larval infestations peaked during summer, whereas summer and spring peaks were noted for the nymphal infestations. The relative importance of cotton rats and deer mice as hosts for immature ticks could be largely, but not completely, explained by cotton rats being more than three times as abundant as deer mice. Attachment site data indicated that differences in grooming behavior also might be partially responsible for the larger infestations observed on cotton rats. Other possible ecological and behavioral explanations of the heavy infestations observed on cotton rats are discussed.