Fifty-one species and subspecies of fleas are currently known in Alaska; 37 species normally occur on mammals, and 14 are associated with birds. With the possible exception of one or two species, none are restricted to the political entity, Alaska. From a study of geography, paleozoology, morphology and taxonomy, the fleas of Alaskan mammals are reported to be derived from three principal faunas. The largest group apparently arose from Eurasia as the mammals migrated across the Bering Land Bridge during the glacial periods of the Pleistocene epoch and earlier. These animals found refuge in a large nonglaciated area of interior Alaska and the Yukon Territory. These fleas are now common to both the Nearctic and Palearctic regions, their hosts generally being Holarctic in distribution. The second group of fleas are Nearctic in origin and have migrated northward along with their hosts as the Pleistocene glaciers receded. A number of the genera in this second group are restricted exclusively to the Nearctic region; however, certain genera are also represented in Eurasia. A third group of fleas arose from the Pacific Northwest. These fleas, like the second group, have spread northward as the Pleistocene glaciers receded. Fleas of this third group now occupy a narrow strip of land, mostly west of the Coast and Alaska Ranges. A study of host associations reveals that distribution of the fleas is not concordant with that of the preferred hosts. For example, of six species as so dated with microtine rodents in the taiga, only two have followed the same hosts into the vast tundra region to the north. Fleas originally thought to be restricted to the Arctic regions are now known to have adapted to hosts within the taiga, albeit the distribution is not an extensive one. These distribution patterns indicate that the Siphonaptera are subject to ecological factors and pressures over and above those which affect the distribution of the mammals and birds upon which they depend for their existence. Additional knowledge is urgently needed in order to understand the biology of the flea, especially in the subarctic and Arctic areas of the world. The data collected thus far indicate that most fleas in the taiga have one or possibly two generations a year. By late winter and early spring, Malaraeus penicilliger dissimilis (Jordan, 1938) is frequently the only species encountered upon the microtine rodents. During July, August, September and October, the small mammals are most abundant. Simultaneously, the flea infestation rates upon the mammals increase, and the number of different species encountered also becomes more varied.