We compared host selection by two subspecies of Abert's squirrel, a highly specialized and selective herbivore. Both subspecies are restricted to stands of ponderosa pine, upon which they are ecologically dependent, but the two subspecies are geographically disjunct and each is closely associated with stands of trees that represent strongly differentiated chemical races. The criteria by which trees were selected as sources of phloem by each subspecies of squirrel included heritable features of xylem oleoresin and phloem. Trees that were potentially available to the subspecies of squirrel in Colorado differed substantially in biochemical features from trees that were available to the subspecies in Arizona; as a result, chemically mediated feeding patterns were distinct between the two squirrel subspecies. Based on multivariate analysis of chemical characteristics, trees utilized by the squirrels for feeding (target trees) differed significantly from control trees (non-target tre es) for each subspecies. However, the discriminant functions that separated target trees from non-target trees within a site generated different patterns of predicted classification when applied to trees at the other site. Vertebrate herbivores that are feeding specialists can exert selection pressures in populations of their host plants, and results suggest that geographically differentiated herbivore-host interactions can produce different sets of selection pressures, which may result in different evolutionary outcomes. Such geographic differentiation is a potentially important evolutionary aspect of chemically mediated mammal-plant interactions.