THE S locus governing gametophytic self-incompatibility in plants is particularly suited to mutation studies. Pollen grains having an unmutated allele are unable to grow through a style carrying the same allele. In contrast ‘mutant’ grains produce pollen-tubes which may overcome the incompatibility barrier. Thus, mutation can be scored simply as seeds produced in normally sterile matings, and then subjected to confirmation by progeny phenotypes. Studies of S locus mutations have been confined to heterozygous plants2–4 as homozygotes are difficult to obtain in many species. Since pollen S phenotype is a gametophytic character, it has been implied that mutation frequencies of alleles in homozygous plants would simply reflect those of alleles in the corresponding heterozygotes. The present study does not support this postulate.