In this paper we present a study of the equilibrium resulting from the balance between recombination and selection against the recombinant phenotypes. The model described in our previous paper, for the phenotypic effects of the character differences involved in heterostyly, is used as a basis for this study. We also analyze the dynamics of populations when conditions change so that some of the recombinant types are at a selective advantage, resulting in the breakdown of heterostyly. A model of the breakdown of heterostyly was first constructed by Crosby (1949) and later elaborated by Bodmer (1960). These authors considered only the spread of long homostyles into a heterostyled population. The case of short homostyles was studied by Dowrick (1956). All these authors assumed that homostyled genotypes suffered a selective disadvantage which, if sufficiently strong, could prevent their fixation. There does not, however, seem to be any direct evidence for such a selective disadvantage to the homostyles. The models studied in the present paper assume only that the products of self-fertilization may be less fit than those of cross-fertilization; this enables us to examine the fate of all possible crossover products within the heterostyly supergene without having to make ad hoc assumptions about the fitnesses of specific genotypes.