About half of the phleomycin-resistant progenies derived from Bacillus subtilis 160 were genetically unstable and yielded morphological, nutritional, and growth-rate mutants at considerably high frequencies. The high mutability was evidently transferred to another strain by the DNA-mediated transformation technique. The phleomycin gene and the mutability- controlling gene appeared to be located near the streptomycin and erythromycin loci.