The tail is covered with stiff hairs that form a short, thin terminal pencil. Eothenomys is perhaps phylogenetically intermediate to Clethrionomys and Microtus, as it possesses some characteristics common to both genera. These animals frequent easily tunneled banks and slopes in both wooded growth and mountain meadows. Their elevational range is from about 1,800 to at least 4,400 meters. They tend to be active both day and night.
Some forms seem to spend more time underground than others.
Burrows open to the surface at frequent intervals, and numerous surface runways may be evident. Surface runways of other animals, such as moles, may also be used. The habits of Eothenomys are presumably like those of Clethrionomys and Microtus. Breeding apparently occurs throughout the year, or nearly so, as pregnant females have been found in the wild from February to October.