Summary Echinococcus multilocularis Leuckart, 1863 (syn. E. sibiricensis Rausch and Schiller, 1954), whose larva causes alveolar hydatid disease in man, is reported for the first time from continental North America. The adult cestodes were collected from foxes at four localities in arctic Alaska, and also from Nunivak Island, which lies off the western coast of Alaska near the delta of the Kuskokwim River. E. multilocularis is apparently widely distributed in Alaska, and may be expected to occur wherever the predator-prey relationship existing between the arctic and red foxes and microtine rodents, particularly the brown lemming, favors completion of its life cycle. The controversy of the past regarding the identity of the cestode causing alveolar hydatid disease is now clarified by the recognition of the coexistence of two species, E. granulosus and E. multilocularis, which differ in host-relationships and morphological characteristics. These species, however, are known to be sympatric only in boreal regions, the distribution of E. multilocularis being restricted by the occurrence of suitable intermediate hosts (rodents of the genera Microtus, Clethrionomys, and Lemmus). Although E. multilocularis is known only from parts of Alaska having an abundant mammalian fauna, it is believed that conditions in southern Canada and the United States are favorable enough to permit its becoming established, should it be introduced via infected dogs from the north.