Abstract: Athyrium filix-femina (Lady Fern) is a species complex composed of at least six varieties worldwide.  Although previously studied at the population level, much remains unknown about the genetic structure of populations and the levels and patterns of genetic variability in this taxon.  As a part of a larger biosystematic study of variation in this globally distributed complex, research was performed on A. filix-femin. var. asplenioides (Southern Lady Fern) to look at patterns of clonal and sexual reproduction within populations.  Data collected by isozyme electrophoresis showed that Athyrium clones vary in size and morphology, exhibiting both phalanx and guerilla structures and have the capacity to form large clonal patches (up to 17m in length).  However, vegetative reproduction plays only a minor role in population structure; most ramets or small clusters of ramets are distinct genets.  Levels of genetic variability (A=2.05, P = 32.6, H = 0.113) were near average of other fern taxa.  The majority of the loci showed conformance to Hardy-Weinberg expected genotype proportions, revealing that the mating system was predominately outcrossing.  Occasional heterozygote deficits were most likely attributable to population substructure; rare heterozygote excesses may reflect selection favoring heterozygotes.  Population substructure was also indicated by hierarchical Fst analysis showing Fst values among subpopulations of the same magnitude (Fst = 0.034) as among populations across the study region.  Evidence of modest population substructure was suggested by spatially clumped occurrences of rare alleles.