Long considered one of the best examples of a moss endemic to the Blue Ridge Escarpment of the Southern Appalachian Mountains, “Mnium carolinianum” has been reduced to synonymy with the widespread pantropical Plagiomnium rhynchophorum. It therefore joins a number of other bryophytes and ferns that represent tropical species disjunct to the moderated microenvironments provided by the deep gorges of these high-rainfall mountains. Our evidence from starch gel electrophoresis of isozymes and sequencing of both chloroplast (rps4 and rpL16) and nuclear (ITS2) DNA supports this taxonomic decision originally proposed by Timo Koponen but largely ignored in recent checklists and floras. A similar situation prevails in Hawaii, where plants previously referred to P. rostratum also proved to be P. rhynchophorum. Both of these species are allopolyploids, expressing fixed heterozygosity at diagnostic loci. Although more extensive sampling of populations and additional genetic markers are desirable, it is clear that P. rhynchophorum combines the genomes of P. maximoviczii and P. integrum, whereas P. rostratum combines the genomes of P. vesicatum and P. integrum. This conclusion is congruent with morphological evidence, which shows the allopolyploids are intermediate or combine diagnostic characters of the progenitor haploids.