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The mountain beaver (Aplodontia rufa (Rafinesque, 1817)), endemic to western North America, is the only extant member of the family Aplodontidae. Limited information on movements and habitat use throughout the species' range is available. We radio-collared 41 mountain beavers to determine home ranges, dispersal, habitat use, and population densities on two managed forest sites in coastal Washington. Both sites were recently harvested for timber. The Donovan site (16.6 ha) was treated with herbicide before seedling planting and the Sylvia site (8.9 ha) was not treated. Mountain beaver home ranges (Donovan: 4.18 ± 0.81 ha; Sylvia: 1.39 ± 0.4 ha) were greater than previously reported in the literature (0.02-0.2 ha). Home ranges (P = 0.009) and core use areas (P = 0.05) on the herbicide-treated Donovan site were larger than those observed on the untreated Sylvia site. Mountain beaver population density declined from 2002 (n = 16) to 2003 (n = 8) on the Donovan site, and reinvasion onto the site after removal trapping was low (n = 7). Population density more than doubled on the Sylvia site from 2002 (n = 21) to 2003 (n = 55), and reinvasion was greater (n = 27) than observed at Donovan. In addition, we documented habitat characteristics centered at 9 Donovan and 10 Sylvia nest locations. Sites were similar in most habitat characteristics, but the Sylvia site had more herbaceous forbs, stumps, and woody debris. Our findings suggest that mountain beaver populations and home ranges are affected by availability of forage, such as herbaceous forbs, after herbicide treatment, as well as by availability of woody cover.