Tick burdens are aggregated on hosts both among potential hosts and within a single host species. The goals of this thesis were (1) to determine factors that influence heterogeneous Dermacentor variabilis burdens on Peromyscus leucopus in the field and (2) to determine if immature D. variabilis could differentiate hosts via chemical cues on two levels; among hosts and within a single host species. Host sex, body mass and habitat were predictors of immature D. variabilis burdens in the field. Larval D. variabilis exhibited no chemical attraction but nymphs did. Nymphal D. variabilis significantly preferred a principal host (P. leucopus) over a non-host (Cryptotis parva) but not an auxiliary host (Microtus ochrogaster). Nymphal D. variabilis could differentiate between P. leucopus based on host body mass and sex in the presence and absence of CO2. This work provides evidence for a two-tiered approach to aggregated parasitism, as mechanisms behind heterogeneities likely differ among potential hosts relative to within a single host species.