The fur is soft and dense. The upper parts are grayish to grayish buff but occasionally, in season, become fulvous brown. The underparts are whitish, often faintly washed with buff on the chest. The relatively well-haired tail is bicolored, dusky above and white below. The ears are large and nearly naked.
Females have six mammae.
The volcano mouse lives on grassy slopes in forests at an elevation of 2,600-4,300 meters. It is nocturnal and is most active shortly after dark. It does not usually make distinct runways.