Angiosperm species that produce unisexual flowers pres? ent the opportunity for separate analysis of the male and female programs for floral differentiation and gametogenesis. A vast literature describes the genetic and physio? logical basis of sex determination in these species. (For reviews, see Westergaard, 1958; Grant, 1975; Frankel and Galun, 1977; Durand and Durand, 1984.) Recently, it has become feasible to pursue the molecular genetic basis of the male and female differentiation programs in certain plant species, as has already been profitably undertaken in several animal species (Hodgkin, 1987, 1989). This article reviews the unisexual flowering schemes found in angiosperms and summarizes available data on the control of floral polymorphism in the monoecious species maize (Zea mays) and in the dioecious species mercury (Mercurialis annua).