By most accounts, Plagiomnium cuspidatum is a typical bisexual, diploid species with n  =  12 throughout its wide range. Beginning with Lowry's (1948) chromosome count of n  =  6 for a collection from northern Florida, however, there have been sporadic reports of haploid or unisexual plants of this species. Unfortunately, none of these cases has been examined closely enough for sexuality, ploidy level, and possible morphological or other differences to be firmly established. Our studies of natural populations from Florida and Georgia reveal that there is a previously undescribed species that differs from P. cuspidatum in sexual condition, ploidy level, morphology, and ecology. This unisexual species has a chromosome number of n  =  6, elliptic leaves with teeth that extend more than halfway down the margin, leaf cells mostly < 25 µm, and a preference for circumneutral soils associated with limestone. Its closest relatives in sect. Plagiomnium are P. cuspidatum and P. acutum, but Nei's (1978) unbiased genetic identities of 0.669 and 0.705, respectively, show that it is strongly differentiated from both (Wyatt & Odrzykoski 1998). Independent evidence of the distinctiveness of these populations from the southeastern United States comes from phylogenetic reconstructions based on sequencing of chloroplast (rps4, rpL16) and nuclear (ITS2) gene regions by Harris (2008).