Genetic relatedness of 14 yeast strains and 2 mold strains was studied by the DNA-DNA hybridization method. The hybridization was performed between mitochondrial-DNA-free, 32P-labeled DNA of Saccharomyces cerevisiae IAM 4009 and cold DNA of other strains. The DNA homology indices deviated considerably even among S. cerevisiae strains having similar GC contents, but, in general, yeast strains known to be able to mate with S. cerevisiae, showed high homology indices (35_??_70%). Other species of Saccharomycetaceae and 6 as-porogenous yeast strains exhibited values of 10_??_20%. The relatedness suggested from these results was confirmed by the competition experiments and also by the hybridization with 32P-DNA of Candida pulcherrima IFO 0561. DNA's of Aspergillus oryzae I and Neurospora crassa IFO 6067 also exhibited low but appreciable homology indices (5_??_7%). These results were discussed from the aspects of phylogenetics and also of gene conservation in microorganisms.