Mating type interconversion in homothallic Saccharomyces cerevisiae has been studied in diploids homozygous for the mating type locus produced by sporulation of a/a/a/alpha and a/a/alpha/alpha tetraploid strains. Mating type switches have been analyzed by techniques including direct observation of cells for changes in alpha-factor sensitivity. Another method of following mating type switching exploits the observation that a/alpha cells exhibit polar budding and a/a and alpha/alpha cells exhibit medial budding.-These studies indicate the following: (1) The allele conferring the homothallic life cycle (HO) is dominant to the allele conferring the heterothallic life cycle (ho). (2) The action of the HO gene is controlled by the mating type locus-active in a/a and alpha/alpha cells but not in a/alpha cells. (3) The HO (or HO-controlled) gene product can act independently on two mating type alleles located on separate chromosomes in the same nucleus. (4) A switch in mating type is observed in pairs of cells, each of which has the same change.