Cystopteris protrusa is a member of the C. fragilis complex and was once considered a variety of C. fragilis. Unlike the related species the stem usually extends past the point of attachment of the current year's fronds and it usually grows on soil (rather than rocks). Neither of these characters is of much use in distinguishing dried specimens consisting of a few picked fronds with inadequate label data. 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.

Cystopteris protrusa is known from easternmost Oklahoma and Minnesota, east to New England and south to Georgia and Louisiana. In Wisconsin it is mostly found in the southern counties.