Asplenium monanthes L. is an apogamous fern with a wide tropical and subtropical distribution that extends into the temperate zone in the southeastern U.S. These populations are quite distant from the nearest neotropical populations, so the circumstances of their origin (i.e. as pre-Pleistocene relicts or by later long-distance dispersal) were investigated using spore and gametophyte morphology and starch gel electrophoresis. The southeastern U.S. populations of A. monanthes can be attributed to multiple long-distance dispersal events during the Pleistocene or Holocene. This recent origin and A. monanthes' small population sizes account for southeastern U.S. A. monanthes' lack of genetic diversity within and in many cases among populations. The source of the various founding spores cannot be conclusively determined, but evidence favors the Caribbean over Mexico. The southeastern U.S. populations of Asplenium monanthes do not warrant distinct taxonomic status. Analysis of genetic structure revealed great diversity in Mexico, so Mexico is suggested as a possible birthplace of allotriploid A. monanthes. The rarity and disjunct nature of the southeastern U.S. populations also prompted investigation into their ecology in the temperate zone and their prospects for continued survival. Asplenium monanthes survives in the southeastern U.S. by inhabiting shaded gorges, sinkholes, and cave entrances that moderate temperature extremes and maintain reliable moisture levels. The populations are very small, so they are subject to environmental and demographic stochasticity and are fixed genetically for relatively few multilocus genotypes. Several historical populations have disappeared in recent decades and several remaining populations appear to be declining as well. These declines may be a natural part of the species' regional population dynamics or they may be cause for alarm. Regular long-term monitoring is required to determine whether the species is truly at risk of local extinction in the southeastern U.S. and if so, which factors are most responsible for this trend. No management intervention is recommended at this time.