If the natural enemy component of biological control is functional in small mammals, then the introduciton of weasels should effectively reduce populations of deer mice and Oregon voles. Seven weasels introduced to an area of forest habitat in coastal British Columbia did not appreciably lower the abundance of mice or voles. However, these natural predators may have contributed to the localized extinction of the vole population on the experimental area. Weasels were generally more abundant in the late summer and early fall and preferred clearcut (logged) successional habitats compared with forested areas. The results of this study are discussed with respect to the futility of poison baiting and related techniques of removing rodent pest populations from forest and agricultural land developments.