Coloration varies, but generally the upper parts are grayish brown and the underparts and feet are silvery gray. The pelage is long and silky. Phenacomys so closely resembles the genera Microtus and Clethrionomys that positive identification may not be possible until the skull and dentition are carefully examined. Unlike Microtus, adult Phenacomys have rooted cheek teeth. Unlike in Clethrionomys, in Phenacomys the inner reentrant angles of the lower molar teeth are much deeper than the outer.
Females have eight mammae.
It seems to prefer open coniferous forests with an understory of heaths or areas of shrubby vegetation on forest borders or in meadows. It is terrestrial and largely nocturnal or crepuscular. It is active throughout the winter, at which time it builds a spherical nest about 150 mm in diameter composed of twigs, lichens, and grass. The nest is placed on the ground between rocks or under a shrub, and from it radiate tunnels out under the snow. The summer nest is built about 100-250 mm underground, and tunnels radiate outward and upward to the surface.