Bacillus subtilis was exposed to ultraviolet light (UV) or X rays, and gene frequency analysis was used to study the location of initiation sites of postirradiation deoxyribonucleic acid (DNA) synthesis. It was found that DNA synthesis resumes primarily from the origin after UV exposure. With X irradiation, the origin is not selectively replicated. Elevated origin-to-terminal marker ratios observed after UV exposure of exponentially growing cells were interpreted as evidence for selective UV resistance of the replicative origin region of the bacterial chromosome. This laboratory has been engaged in studies aimed at defining the quantitative and regulatory aspects of chromosome replication after ultraviolet light (UV) or X-ray challenge to bacterial cells. By using a combination of density and radioisotopic labeling techniques, it was previously shown (4) that, when bulk deoxyribonucleic acid (DNA) synthesis re