A sporophytic self� incompatibility system, such as occurs in Carthamus jlavescens Spreng., was studied by computer simulation. Equilibrium gene fre-quencies in an infinite population were estimated for three-allele and six-allele systems and found to be independent of initial frequencies. In a species existing as a series of more or less isolated small populations genetic drift caused rapid loss of alleles. Maintenance of the S allele system was enhanced by increased population size, and particularly by migration, by which alleles lost from one local population could be re-introduced from adjacent populations. Hard seed carryover had a lesser effect.