Completely aquatic and gilled throughout life. The mudpuppy is the largest member of the genus Necturus
which also includes waterdogs. All mudpuppies and waterdogs have bushy external gills, two gill slits, a laterally compressed tail, and four toes on front and hind feet. Adult mudpuppies are 20-49 cm total length. Dorsal coloration varies from rusty brown to gray or black with bluish black spots or blotches. A dark stripe occurs on the side of the head, passing through the eye and sometimes extending down the side. The venter is white, gray, yellow, or brown, sometimes with dark spots. Sexually mature males can be distinguished by the swollen cloaca and pair of enlarged cloacal papillae that project posteriorly (Pfingsten and White 1989; Petranka 1998). Hatchlings are 14-15 mm snout to vent length (21-25 mm total length). Juvenile coloration is quite striking, consisting of a dark middorsal stripe bounded by two light yellow stripes. A dark band occurs below the yellow stripes. Juvenile color pattern becomes more simlilar to adults with age, beginning at 13-15 cm total length (Shoop 1965; Petranka 1998).
Two subspecies are currently recognized which differ in geographic distribution (see below) and coloration. The mudpuppy (N. m. maculosus, sometimes called the common mudpuppy) is rusty brown to grey dorsally, with a gray venter that ranges from unspotted to densely spotted. The Red River mudpuppy (N. m. louisianensis, sometimes called the Louisiana mudpuppy or waterdog) is light yellowish brown to tan dorsally, sometimes with a dark dorsal stripe bordered by lighter stripes. The back and sides of the belly have large dark spots or blotches, but the midline of the venter is light colored with no spots (Conant and Collins 1991; Petranka 1998).
A genetic survey found little divergence between populations of mudpuppies from Massachusetts, Minnesota, and North Carolina (Ashton et al. 1980). Necturus beyeri, N. alabamensis, and N. maculosus are relatively closely related (Guttman et al. 1990).