A recessive temperature-sensitive mutation of Saccharomyces cerevisiae has been isolated and shown to cause an increase in ploidy in both haploids and diploids. Genetic analysis revealed that the strain carrying the mutation was an aa diploid, although MNNG mutagenesis had been done on an a haploid strain. When the mutant strain was crossed with an alpha alpha diploid and the resultant tetraploid sporulated, some of the meiotic progeny of this tetraploid were themselves tetraploid, as shown by both genetic analysis and DNA measurements, instead of diploid as expected of tetraploid meiosis. The ability of these tetraploids to continue to produce tetraploid meiotic progeny was followed for four generations. Homothallism was excluded as a cause of the increase in ploidy; visual pedigree analysis of spore clones to about the 32-cell stage failed to reveal any zygotes, and haploids that diploidized retained their mating type. An extra round of meiotic DNA synthesis was also considered and excluded. It was found that tetraploidization was independent of sporulation temperature, but was dependent on the temperature of germination and the growth of the spores. Increase in ploidy occurred when the spores were germinated and grown at 30 degrees, but did not occur at 23 degrees. Two cycles of sporulation and growth at 23 degrees resulted in haploids, which were shown to diploidize within 24 hr when grown at 30 degrees. Visual observation of the haploid cells incubated at 36 degrees revealed a cell-division-cycle phenotype characteristic of mutations that affect nuclear division; complementation analysis demonstrated that the mutation, cdc31-2, is allelic to cdc31-1, a mutation isolated by Hartwell et al. (1973) and characterized as causing a temperature-sensitive arrest during late nuclear division. The segregation of cdc31-2 in heterozygous diploids was 2:2 and characteristic of a noncentromere-linked gene.