AbstractBreeding‐system data have been available for a large number and diverse array of angio‐sperms for a relatively long time. In contrast, breeding systems of ferns and their allies (pteridophytes) have only recently been examined, and breeding‐system data from natural populations of sporophytes are still lacking for pteridophytes representing many life‐history strategies. Few studies, for example, have examined breeding systems of tropical pteridophytes, and no breeding‐system data are available for tree ferns. We therefore examined the breeding systems of three species of tree ferns from Costa Rica,Alsophila firma(Cyatheaceae),Cyathea stipularis(Cyatheaceae), andLophosoria quadripinnata(Lophosoriaceae) using enzyme electro‐phoresis. Genetic data were used to estimate intragametophytic self‐fertilization andF, the fixation index. Analysis of genetic data indicates that the gametophytes of these three species predominantly cross‐fertilize; all three species would be characterized as outcrossers. However, some population‐to‐population variation in breeding system was detected in all three species. Outcrossing also typifies a diverse array of temperate ferns. Thus, despite the potential for self‐fertilization, outcrossing appears to characterize the majority of pteridophytes representing a variety of evolutionary lineages, life‐history strat