Rare diploids formed by sterile mutants have been studied by tetrad analysis. Sixteen classes of mutants representing at least five distinct genetic loci have been defined. One group of mutations, isolated only in alpha, maps at the mating-type locus, while none of the others shows any linkage to mating type. Some of the mutations are nonspecific for mating type, while others act only on a or alpha. In addition, mutations were found that prevent sporulation when heterozygous in diploids. These appear to be mutations of the mating-type alleles.