Gametophyte morphology and development of the federally listed American hart’s tongue fern Asplenium scolopendrium var. americanum L. was examined and compared to the old world variety Asplenium scolopendrium var. scolopendrium. The North American variety is critically threatened and many populations in the Southeastern US have gone extinct while the number of plants in all northeastern populations is plummeting. The ultimate goal of this work was to compare gametophyte biology of these two varieties to understand better potential characters that may help separate plants in North America from those in the old world and generate a better picture of the biology of the North American populations. The two varieties differed significantly in gametophyte ontogeny, morphology, and propensity for sexual and asexual reproduction. Gametophytes of the American variety germinated earlier, grew significantly slower, and produced sporophytes much later (166 d vs. 119 d) than the old world variety. In addition, A. scolopendrium var. americanum produced copious gametophytic outgrowths that were capable of developing into functional, independent thalli while A. scolopendrium var. scolopendrium does not. The combined data set aids in understanding the origins of the American variety as its gametophyte biology is similar to several other taxa which now persist in climatically–moderated rockhouse habitats in eastern North America.