The great diversity of mating systems exhibited by flowering plants has attracted the attention of botanists for nearly two centuries, and the causes and consequences of this diversity have been extensively discussed. Until quite recently, however, little attention has been paid to the mating systems of ferns and other homosporous plants. Klekowski & Baker (1966) were among the first to address the problem, proposing that most sexual reproduction in fern populations is a result of self-fertilization within a single gametophyte. Although a number of studies on the distribution of lethal alleles in fern populations appeared in the 1970's (e.g. Klekowski, 1973, 1979; Lloyd, 1974), it is only within the last five years that data on genotypic diversity in homosporous plants have begun to accumulate rapidly (e.g. Haufler & Soltis, 1984; Gastony & Gottlieb, 1985; McCauley et al., 1985; Soltis & Soltis, 1986, 1987). With these data it is now possible to describe the mating system of a variety of homosporous plants (Holsinger, 1987). The sperm and eggs produced by a single gametophyte are genetically identical, except in the unlikely event of a somatic mutation within the gametophyte. Thus, if a sperm fertilizes an egg produced by the same gametophyte, the resulting sporophyte will be homozygous at every locus. In contrast, self-fertilization in seed plants results in only a halving of heterozygosity in each generation. Thus, self-fertilization in homosporous plants is the most extreme form of inbreeding that can occur. In terms of the genetic consequences, the only systems that are directly comparable are certain forms of asexual reproduction in insects (Stalker, 1956; Carson et al., 1969; Ochman et al., 1980). Self-fertilization in homosporous plants, therefore, provides an excellent testing ground for theories of mating system evolution. Studies of the mating system in homosporous plants have commonly distinguished between three types of mating events (Lloyd, 1974; Klekowski, 1979): (1) outcrossing, the cross-fertilization of gametophytes derived from the spores of different sporophytes, (2) intergametophytic selfing, the crossfertilization of gametophytes derived from the spores of a single sporophyte, and (3) intragametophytic selfing, the self-fertilization of a single gametophyte. The natural distinction is, however, between matings involving a single gametophyte and matings involving different gametophytes. It is not the genealogical relationship of two gametophytes that determines the chances that a mating occurs between them, but their proximity to one another. If intergametophytic selfing occurs, it occurs as a result of correlated spore dispersal. The spores of Asplenium lepidum, for example, are dispersed as a unit in