As the flea fauna of mammals of northern North America becomes better known, its very close affinity with that of temperate and northern Asia becomes more apparent. Many of the genera are holarctic and are associated with holarctic genera of mammals. In a few instances, the fleas of the Old World and the New World are so similar in morphological details and host association that they are regarded as subspecies. For example, Amphipsylla sibirica (Wagner), Malaraeus penicilliger (Grube), Megabothris calcarifer (Wagner), and Hoplopsyllus glacialis (Taschenberg) have representatives in both the nearctic and the palaearctic regions. To this list must now be added Catallagia dacenkoi Ioff, a parasite of Microtinae; this species was described originally (Ioff, 1940, pp. 216-217) from the Altai Mountains of central Russia, and a new subspecies has now been discovered in northern Canada and Alaska.