This two-part study of butterflies in the genus Euphydryas addresses questions related to habitat suitability. Part I tests the hypothesis that food plant preferences evolve in response to ecological factors, such as plant phenology, abundance, year-to-year persistence, and mammalian herbivory, rather than to biochemical properties specific to the host. Larval feeding experiments clearly demonstrate that monophagous Colorado E. editha are not specialized digestively on the oviposition plant. Important ecological differences among potential hosts, however, are evident. Host-specificity appears to be governed by the phenological availability of the plant. In part II the transfer of larvae from a montane Wyoming population of E. gillettii south to locations in the central Colorado mountains has resulted in the establishment of at least one Colorado population. This indicates that the previous absence of the species from Colorado was the result of its inability to cross the barrier of the Wyoming Basin, not the ecological unsuitability of Colorado habitats. This supports the contention that Euphydryas do not easily disperse long distances.