There is generally a white or pale-colored spot just below the ear. The large grayish ears are practically naked. The tail is, however, scaly and poorly haired. Colomys has a light and slender body form. The feet are elongate, and the four fingers are long.
Females have eight mammae.
The incisor teeth are not grooved and are flat or slightly concave on the front surface. In Colomys, however, both the upper lip and the chin are swollen and cover the incisors. This modification may be associated with the semiaquatic lifestyle of Colomys. It is nocturnal and constructs short tunnels in stream banks. It swims rapidly with powerful thrusts of its long hind legs.