Ten adult Quinqueserialis quinqueserialis specimens were removed from the intestine of a naturally infected muskrat, and scanning electron microscopy was used to study the morphological characteristics of the trematodes. The mature trematode, which was easy to recognize by the monostome holdfast organ, with no anterior cone, measured 2200-2500 μm in length by 900-1050 μm in width. The body was elongated and tapering at the anterior end, but the posterior end was rounded, and in some specimens was slightly truncated. The mouth opening lay at the anterior end and was surrounded by the oral sucker, which was round, small to medium in size, and subterminal. The tegument of the rim and inside of the oral sucker was smooth and had two types of papillae, domed and rosette papillae. Around the oral sucker, tegument was covered with sharp, pointed spines. The common genital pore was located on the median line of the body, posterior to the oral sucker. The cirrus had smooth tegument at the base and was armed with numerous conical spines throughout its length. The ventral surface was concave and provided with five distinct longitudinal rows of ventral papillae, which extended from the anterior to the posterior end of the body. Each row consisted of 15 to 20 papillae, making 81 to 88 papillae in all. These papillae were variable in size. In most specimens, the papillae were simple knob-like structures, but in some cases, they appeared to be bi- or trifurcate. The tegument at the base of each ventral papilla showed minute spiny pattern, but it was smooth or folded on top and had small rosette and ciliated papillae. Tegument at the edges of the worm was smooth in the mid-parts, spiny on lateral parts, and included rosette papillae. The dorsal surface of the worm was smooth and slightly convex, and the tegument was provided with two large domed papillae in one third of the anterior end of the dorsal part, few thick spines in the mid-part, and excretory pore at the level just posterior to the end. No spines or papillae were seen around the excretory pore.