A robust, semiaquatic rodent with a large head, small ears, small front legs having feet with unwebbed toes, large hind legs having feet with webbing between the first 4 toes, and a round tail. The tail is 12-18 inches in length, is scaly and scantily haired. The general body color is brownish.
Similar species: The nutria somewhat resembles a large muskrat or a small beaver but can easily be distinguished by the round tail (the muskrat's is vertically flattened; the beaver's is horizontally flattened). The tail shape is difficult to see when the animal is swimming.