Dynamics of small mammal populations and the prevalence of antibodies for hantavirus were determined in six locations in central and western Montana (USA). Eighteen live-trapping grids were trapped monthly from June through September 1994. Deer mouse (Peromyscus maniculatus) populations ranged from 0 to over 90 on one-hectare grids. Our bleeding technique had no apparent effect on survival of deer mice. Deer mice, meadow voles (Microtus pennsylvanicus), and sagebrush voles (Lagurus curtatus) were seropositive. Thirty-eight (8%) (range, 0% to 30%) of 471 deer mice were seropositive for hantavirus antibodies. Seropositive mice were older and had lower monthly survival rates than seronegative deer mice. We found no relationship between prevalence of hantavirus antibodies and population density.