The coarse guard hairs (outer fur) are dark brown and glossy. The dense underfur is a rich brown at the tip on the back and shifts to gray at the base, pale buff belly. Resembles a small muskrat, from 381 - 546 mm (15 - 21.5 in) in total length. The tail is round instead of flattened on the sides like muskrats.
Breeds throughout the year but peaks in late autumn. After 26 - 29 days gestation, 1 - 4 young are born; females producing from 4 - 6 litters per year. At 14 - 18 days of age young are able to care for themselves and are weaned by 21 days, reaching sexual maturity (able to reproduce) at 90 - 100 days.
Roundtail muskrats inhabit shallow freshwater marshes that have sandy bottoms and dense aquatic vegetation. This species builds dome-shaped lodges of aquatic grasses and plants, attaching them to emergent vegetation over a bed of sphagnum moss. The lodge is frequently built at the base of a cypress tree or clumps of brush. The interior of the lodge houses a nest chamber lined with fine dry grasses, and generally two entrances which exit underwater. Feeding platforms are also built near the lodge, consisting of a pad of plant material. They are elevated slightly above the waters surface, and contain one or two plunge holes for quick escapes from predators. It is most active shortly after dark and just before dawn. Aquatic grasses make up the bulk of its diet but stems, roots, and seeds are also eaten. Major predators of the Roundtail Muskrat are herons, owls, hawks, snakes, and bobcats. Population densities can range from 25 - 100 individuals per acre in good habitat. This species is not trapped commercially for pelts and is a protected species in Georgia (see Conservation Status).
Endemic to the extreme southeastern corner of Georgia and peninsular Florida.
The roundtail muskrat is listed as a Threatened species in the state of Georgia and is protected from trapping and hunting.
Roundtail muskrats are distinguished from the muskrat by two characters: they are noticeably smaller and have a round tail, not a laterally compressed tail.