Nemophila menziesii typically bears perfect, protandrous. self-compatible but predominantly outcrossed flowers. In one population of Nemophila menziesii var. intermedia at Point Reyes, California, approximately 25% of the plants are completely male sterile. The flowers of these pistillate plants possess only small rudiments of stamens and lack anthers entirely. This phenotype is constant in nature as well as under cultivation. Male sterility appears to be inherited cytoplasmically, but there is at least one nuclear fertility-restoring allele in the population. The fertility-restoring allele may be at a single multiallelic locus, recessive to one nonfertility-restoring allele and dominant to another nonfertility-restoring allele, or there may be fertility-restoring alleles present at two or more loci. The population carries a significant load of deleterious recessive alleles at other loci, as evidenced by floral abnormalities and partial pollen abortion in some of the progeny from selfed hermaphrodites. Male sterility appears to be maintained in this population because the obligately outcrossed male steriles produce progeny which are more fit than the progeny produced from selfed hermaphrodites. Self-fertilization of hermaphrodites is probably higher in this population than in most populations of the species because the oligolectic-pollen-collecting bees, which are the normal pollinators of N. menziesii, are not present at this locality. Gynodioecy probably became selectively advantageous when the outcrossing ancestors of this population were subjected to drastic increases in the rate of self-fertilization owing to changes in pollinator fauna.