Female small mammals are often territorial. Two hypotheses explaining territoriality of females predict that females should exclude competing females from territorial boundaries. For water voles, these hypotheses allow either static territoriality, in which the geographical locations of territories remain constant, or drifting territoriality, in which the locations of territories constantly shift. If females defended entire territories over short periods, gains and losses of habitat would be actively competed for, potentially resulting in static territoriality. We attempted to discern whether water vole territories drift, and why, and whether territories at high densities are more static, by analyzing patterns of short-term and long-term range use at different population densities. Water voles were radiotracked at 2 study sites, 1 supporting a high population density and 1 supporting a low population density. Range sizes of females did not reach asymptotes with increasing observations at either site, and the geographical locations of their ranges constantly shifted via gains and losses of habitat. The degree to which territories of females shifted was larger at the low-population-density site. These range-use patterns could be partially explained by mating behavior in which males and females interacted closely for extended periods, potentially leaving areas of the females' territories undefended. We conclude that female water voles exhibit drifting territoriality, that this may partially result from inherent flexibility in their territories due to mating behavior, and that the degree to which territories drifted was smaller at higher density.