Cystopteris tenuis was once considered to be a variety of C. fragilis and is similar in appearance. The pinnae are more likely to be curved in shape and angled toward the tip of the frond than is the case for C. fragilis. See steps 60-64 in the key to distinguish the species, or refer to the Flora of North America vol. 2 for a more thorough treatment.

C. tenuis is known from the northeastern U.S.and eastern Canada, west to Nebraska and Oklahoma and south to North Carolina. It is widely distributed in Wisconsin, but is most common in the southern half of the state. It is found on both rocks and soil.