Small mammals were live trapped on four grids in the taiga woods near Fairbanks, Alaska. Populations were censused at 2—wk intervals during the snow—free months and 4—wk intervals when snow was on the ground. One Microtus oeconomus population reached a peak of 73—83 animals/ha in 1969. Numbers declined over winter and no Microtus were captured until 1972 when the population density began to increase. A sympatric population of Clethrionomys rutilus did not show a 3—4 yr cycle and reached densities of 52—62 animals/ha in the autumn of each year of the study. Live trap survival data and sumilatneous climatological data indicate that overwinter survival was high for both species. Survival was good for Microtus ♀ ♀, while Clethrionomys ♀ ♀ had several periods of poor survival. In ♂ ♂, periods of low survival occurred during population increase and high density. These periods of low survival did not occur at the same time in both species. Nestling survival was poor in both microtines during population increase in 1969, and remained low for the whole breeding season for Microtus. Both species experienced weight losses during late summer and fall periods. Live trapping indicated that this decrease was due to individual weight losses that were as great as 35% over a 3—mo period. The C. rutilus population in the Fairbanks area had a broad niche in that it was found in many habitat types and morphological, physiological and ecological characteristics were variable. The M. oeconomus population, on the other hand, had a narrower niche and was more specialized than the C. rutilus population. It is hypothesized that niche breadth and variation in characteristics of the species are a determining factor for microtine cycling. Cyclic behavior should not be thought of as a product of latitude, or a universal characteristic of small microtines, but as an adaptation to a less variable niche.