Evidence is presented for the isolation of DNA having a molecular weight of 31 ± 5 × 108 from Bacillus subtilis strain 23 Thy− cells. This accounts for the total DNA content of the genome. Single-strand and double-strand breaks induced by 60Co gamma rays were measured by sucrose density centrifugation. The occurrence of repair of single-strand and possibly double-strand breaks at 0 °C is inferred from survival data and the effects of pre- or post-irradiation heating.