Unlike most animals which have individuals with separate male and female sexes, plants exhibit a great range in breeding systems. Most plant species are hermaphroditic with male and female function in the same flower, but many other species, including a number of important crop plants, have separate male and female flowers on the same plant or consist of populations of separate male and female plants (dioecy). Many different hypotheses have been proposed to explain the diversity of breeding systems in plants and the evolution of dioecy. One major argument suggests that dioecy has evolved to avoid inbreeding depression. Inbreeding depression occurs when closely related individuals mate and the resulting offspring show reduced vigor and ability to reproduce. If inbreeding depression is a factor, then one should expect to find both high levels of inbreeding self-pollination occurring as well as high levels of inbreeding depression in populations that are evolving dioecy. Although there have been many theories proposed, very little empirical data on this question is available. We are examining the role of inbreeding depression in the evolution of dioecy in the Hawaiian plant genus Schiedea. This group is apparently evolving dioecy and exhibits a range of breeding systems in very closely related species. This research is of special urgency because many important intermediate species from the fragile Hawaiian ecosystems are near extinction because of habitat destruction, grazing introduced by herbivores and competition with introduced plants. Knowledge of factors promoting changes in plant breeding systems has many potential breeding applications to practical problems in agriculture and forestry. Inbreeding depression is well known to influence yield in many economically important crop plants, such as corn. Knowledge of factors causing dioecy may lead to greater ability to manipulate sex ratios in plants and is of tremendous value where yield depends upon the production of fruit or seed by the females as is the case of many crop species. Understanding the basis of unisexaulity will also be important in production of hybrid strains, since female individuals are easily hybridized without the necessity of time-consuming costly emasculation of hermaphroditic flowers.