Isozyme and plastid DNA analysis prove that true A. ceterach occurs on the Canary Islands, in addition to A. aureum and an octoploid taxon. Combining morphological and cytological observations leads to correct determination, but the exospore length alone also allows reliable identification of these Canarian species. Our allozyme data suggest that the Canarian A. ceterach population is not completely genetically isolated from the European ones. The holotype of Ceterach aureum var. parvifolium, formerly regarded as an octoploid taxon, proved to be A. ceterach, leaving the octoploid without a correct name. The recently described A. octoploideum shows monomorphic, presumably fixed heterozygosity for a combination of the patterns seen in A. ceterach and A. aureum at four loci (Aat, Skdh, Me, and Pgi-2) confirming its allo-octoploid nature. It most probably originated by chromosome doubling in a tetraploid hybrid between A. aureum and A. ceterach or via the union of their unreduced gametes. Pgi-2 indicates multiple origins of the allo-octoploid, implicating recurrent gene flow from tetraploids to octoploids.