Mutants defective in mating-type conversion were isolated from ascospores of a perfect homothallism strain having the HO HMRaHMLalpha genotype. Eighty mutants, including 11 temperature-dependent mutants showing a or alpha mating potency, were isolated from 10,050 colonies derived from spores mutagenized with ethyl methanesulfonate. Of the 80 mutants, 48 were tested by crossing with an ho HMRaHMLalpha heterothallic and an HO HMRaHMLalpha homothallic strain as standards. The results allowed their division into seven classes. Mutants in the first class were due to mutation of HO to an ho allele. The second class of mutants, csm, lacked the HO function as the result of a mutation unlinked with the HO locus. Mutants in the third and fourth classes were defective in HMRa and HMLalpha gene functions, respectively. Mutants in the fifth and sixth classes were due to a mutation at the mating-type locus showing insensitivity to the function of the homothallic genes. Mutants in the seventh class lacked the expression of mating type as the result of a mutation unlinked to the mating-type locus. All the temperature-dependent mutants lost the mating potency at permissive temperature (25 degrees ). One belonging to the last class of mutants was inferred to be a mutation at a locus necessary for the expression of a mating type at restricted temperature (35 degrees ).