With its streamlined body, small, almost inconspicuous ears, and large, paddle-shaped hind feet, the Mexican water mouse is well adapted to its semi-aquatic lifestyle. The hind feet are also partially webbed and are fringed with hair, giving extra propulsion in the water, and the long, furred tail, which is longer than the length of the head and body, may also aid in swimming (2) (4) (5) (6). The short, dense, glossy fur is dark brown on the upperparts, often with a few longer, silvery outer hairs, and silvery white on the underparts, while the tail is also dark above and white below. The muzzle is quite blunt, with thin, stiff whiskers, and the eyes are tiny. Juveniles are reported to be slate grey in colour (4).
The Mexican water mouse is very similar in appearance to crab-eating rats of the genus Ichthyomys, but can be distinguished by its smaller size and by having four rather than five pads on the palm of the forefoot (2) (4) (5) (6).
- Head-body length: 9 - 14 cm (2)
- Tail length: 9 - 17 cm (2)
- c. 88 g (3)