During a two year study on the relationships between small-mammals and D. variabilis larvae and nymphs, 739 captures of 427 individual animals were examined for ticks. Captures represented eleven species of mammals: Clethrionomys gapperi (Vigors), Lepus americanus Erxleben, Microtus pennsylvanicus (Ord), Mus musculus Linnaeus, Peromyscus maniculatus (Wagner), Sorex cinerius Kerr, Spermophilus franklinii (Sabine), Spermophilus tridecemlineatus (Mitchell), Tamias striatus (Linnaeus), Tamiasciurus hudsonicus (Erxleben) and Zapus hudsonius (Zimmerman). The most frequently captured species were C. gapperi, M. pennsylvanicus, P. maniculatus, S. franklinii and Z. hudsonius, and except for S. franklinii were also the most frequently infested with larvae and nymphs. The dominant host of D. variabilis larvae and nymphs was the red-backed vole, C. gapperi which produced 42.6% and 60.5% of all larvae collected in 1979 and 1980 respectively, and over 85% of all nymphs collected during both years. Peak larval activity occurred between the last week of May and the middle of June and peak nymphal activity occurred in July. Peaks were four to five weeks apart... First cycle voles were predominantly infested with larvae, and individuals from the second cycle were infested with nymphs. Individuals caught between the two cycles infrequently carried small numbers of both larvae and nymphs. Third cycle voles were very rarely infested... Nymphal aggregation was not as great as for larval aggregation. The distribution of nymphs overlapped that of larvae of each year and occupied a greater area. The distribution of nymphs was highly correlated to that of C. gapperi. The prevalence of nymphal infestations in the C. gapperi population was always higher than for larval infestations. The intensity of nymphal infestations was similar to or less than that for larval infestations.