Earlier studies of the Cystopteris fragilis complex led to a prediction that a diploid variety might exist. Such a diploid has now been found, with n = 42, in terrestrial habitats in southern Michigan. The diploid is var. protrusa Weatherby; but a tetraploid in the same region is var. mackayii Lawson. Studies of these two taxa revealed some distinctions not heretofore emphasized, and they may be separated by the following ensemble of characters: (a) internodal length of stems; (b) coloration of petioles; (c) texture of laminae; and (d) cutting of leaf-blades. Since the taxa are well isolated by their different genomes, and since they are readily distinguished morphologically, they may constitute distinct species.