The natural history and ecology of the monarch butterfly (Danaus plexippus L.) have been studied extensively (Ackery and Vane-Wright, 1984). The nearly cosmopolitan monarch is best known for the massive migration undertaken each autumn by North American populations. This event culminates with the butterflies' aggregation into huge overwintering colonies in the Transvolcanic Range of central Mexico and along the coast of California. The monarch is the only temperate representative of an otherwise exclusively tropical subfamily of the Nymphalidae, and although it shares the danaine susceptibility to freezing temperatures in all life stages (Calvert et al., 1983; Anderson and Brower, 1988; Masters et al., 1988), it has been able to exploit the extensive food plant resources of temperate North America through the evolution of this migratory behavior. The selective advantage of seasonal range expansion to utilize large populations of temperate milkweeds (Asclepias species) is regarded as the primary driving force of this unique strategy (Brower, 1977; Young, 1982). The range of the monarch in North America is divided by the Rocky Mountains into eastern and western populations, which retreat to separate refugia each autumn. Monarchs east of the Rocky Mountains migrate to Mexico, while those west of the Rockies migrate to the Pacific coast (Brower, 1985). Marked eastern butterflies released in Idaho were recovered in California, demonstrating environmental, rather than genetic, control over the migratory behavior (Urquhart, 1987). Current gene flow between eastern and western populations has been assumed from the apparent lack of wing-length differentiation and purported range contact in the northeastern Rocky Mountains (Urquhart and Urquhart, 1977; Urquhart, 1987). However, data demonstrating such gene flow have not been published, and the paucity of records of substantial monarch populations in the Great Basin or the northern Rockies suggests that mixing between the eastern and western migratory populations occurs rarely if at all. Whether or not, and to what degree genetic differentiation may have accumulated between these populations, is particularly relevant to understanding the evolution of the migratory phenomenon, and the mon-