The ferns are an ancient group of vascular plants that have yielded a staggering array of systematic problems. Among fern classifications, the number of genera in some families has ranged over 10-fold, and some genera have been treated in up to five different families. Three main groups of leptosporangiate ferns have been recognized; the adiantoid, polypodioid, and dennstaedtioid lines. To clarify relationships among genera and families of dennstaedtioid ferns, we sequenced 1320 bp of the chloroplast gene rbcL from 45 species representing 13 families. Sequence divergence for rbcL averaged 0.9% among species within genera, 10.3% among genera within families, and 14.8% among families, suggesting that the data are appropriate for phylogenetic analysis at the generic and familial levels in ferns. Maximum parsimony analysis resulted in four shortest trees of equal length. The strict consensus tree supported many aspects of previously published hypotheses of relationship based on morphological and cytological variations. For example, the tree ferns (which form a single clade) and Hymenophyllaceae appear as sister groups to the dennstaedtioid ferns on all shortest trees. However, Polypodiaceae and adiantoid ferns, groups traditionally considered separate from the dennstaedtioid families, emerged within the dennstaedtioid clade. This analysis also suggests relationships of some problematical genera, such as Monachosorum, Calochlaena, and Lonchitis. Examination of additional DNA sequences of nuclear genes and developmental studies are needed to evaluate further the relationships suggested by phylogenetic analysis of rbcL sequence data.