Home range size was estimated in two non-cycling populations of Microtus ochrogaster (Wagner) in Northeastern Colorado. Home range length was linearly and negatively correlated with population densities, and positively correlated with the instantaneous rate of increase of both studied populations. These results suggest that factors reflected in the size of the home range may be responsible for the population regulation. The amount of herbage biomass or the biomass of the vole's preferred food were not correlated with either population size or the size of the home range. A negative correlation existed between the instantaneous rate of increase and population density.