S U M M A R YThe DNA homology relationships of 25 micrococci (15 strains of Micrococcus, eight strains of Sarcina and two strains of Staphylococcus) were studied by the deoxyribonucleic acid hybridization method using nuclease S,, an endonuclease specific for single-stranded DNA molecules. Nineteen of the strains were classified into three groups. De Ley et al., 1973). The DNA hybridization method using nitrocellulose membranes is common today, but this method has the disadvantage that DNA fixed on nitrocellulose membranes is released during incubation at high temperatures (Okanishi & Gregory, I 9 7 0 ) . Recently, nuclease S, isolated from Aspergillus oryzae, which is specific for single-stranded nucleic acid (Ando, I 966), was successfully used in improving the nitrocellulose-membrane DNA hybridization method. This endonuclease digests only the single-stranded DNA portion that has not hybridized. Crosa, Brenner & Falkow (I 973) studied the nucleotide sequence relatedness between homologous and heterologous chromosomal and plasmid DNA by the DNA hybridization method with nuclease S, and showed that this method permitted an accurate, reproducible and rapid determination of the extent of polynucleotide sequence homology. In this study, we examined the DNA homology relationships, by DNA hybridization followed by nuclease S, digestion, among species of the genus Micrococcus, including aerobic Sarcina strains which had been proved to belong to the genus Micrococcus by Kocur & Martinec (1965) and Hubalek (1969). 7-2