Myocastor coypus looks like a large, robust rat. Its body is highly arched, and the head is large and almost triangular. The ears and eyes are small and are located in the upper part of the head. The incisors are broad, with orange-pigmented anterior surfaces. The legs are short. The hind feet are much longer than the forefeet, and contain five digits; the first four are connected by webbing, and the fifth is free. The forefeet have four long, flexible, unwebbed digits and a vestigial thumb. The tail is long and rounded. Females have four pairs of thoracic mammae that are situated well up on the sides of the body. The pelage consists of two kinds of hair, soft dense underfur, and long coarse guard hairs that vary from yellowish brown to reddish brown. The underfur is dark gray, and it is denser on the abdomen. The chin is covered by white hairs, and the tail is scantily haired.

Males are generally larger than females. The length of the head and body is 521 mm (472 - 575), and the length of tail is 375 mm (340 - 405).

Gosling (1977), Nowak (1991); Woods et al. (1992).

Range mass: 5.000 to 10.000 kg.

Range length: 472 to 575 mm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: male larger