We studied factors influencing home-range size in fluctuating populations of Microtus ochrogaster and M. pennsylvanicus in alfalfa, bluegrass, and tallgrass habitats over a 25-year period in east-central Illinois. Preferred food availability for both species was greatest in alfalfa and least in tallgrass, lesser during winter than other seasons in all 3 habitats, and greater in alfalfa during winter than in the other 2 habitats. Vegetative cover was sparse, especially during winter, in alfalfa and dense year-round in bluegrass and tallgrass. Movement distances of M. ochrogaster tended to be smaller in alfalfa than in bluegrass and tallgrass; movement distances of neither vole species differed between bluegrass and tallgrass. Within alfalfa, movement distances of both species were smaller during winter, when cover was sparse and food availability low. No seasonal difference was found in movement distances of either species within bluegrass and tallgrass, where cover was dense year-round, but food availability was low during winter. Movement distances of M. ochrogaster were not affected by supplemental feeding in bluegrass and tallgrass; those of M. pennsylvanicus were smaller in supplementally fed tallgrass. We conclude that cover, as an indicator of risk of predation, influenced home-range sizes of both species more than did food availability.