Population dynamics of Clethrionomys and Peromyscus were studied in northern Vermont (March 1963 to July 1966) and central Connecticut (July 1964 to August 1967; supplemented by spot-check data from July 1962 to August 1963 and September 1968). The Connecticut study area included swamp and upland habitats; the Vermont area included only uplands. P. leucopus and C. gapperi displayed an annual cycle of abundance in the Connecticut swamp. C. gapperi and P. maniculatus in Vermont and P. leucopus in the Connecticut uplands displayed periodic peaks of abundance (approximately 2 to 3 year intervals in Connecticut). Greater fluctuation in food supply in the uplands than in the swamp appeared responsible for such differences in annual abundances of Peromyscus and Clethrionomys, at least in Connecticut. P. leucopus population densities were much lower in the swamp than in the upland; lower food availability in swamps as compared to uplands is assumed the primary factor involved. Survival of C. gapperi was slightly greater than that of P. leucopus; this may account for higher overall densities of C. gapperi observed in this study. Seasonal trends in survival were the same for both species with survival highest during the winter months. Survival of P. leucopus in the swamp was only slightly less than that of the upland population. Survival of C. gapperi in a coniferous area of the swamp was greater than that in a deciduous area, possibly due to differences in diurnal avian predation. There was a negative correlation between population density and survival of C. gapperi; a similar, but nonsignificant, trend was observed for P. leucopus. Reproduction of both species reached a peak in May to July; lowest levels of reproduction were observed in January. There was no correlation between population density and reproduction of either species. Length of the previous winter nonbreeding season appeared to be the primary factor influencing population densities reached a given year by both species.