A deficiency in DNA polymerase I increased the ultraviolet (UV) radiation sensitivity of a uvrA strain of Escherichia coli K-12 when plated on minimal growth medium. The slope of the survival curve for the uvrA polA strain was 2.0-times greater than that for the uvrA strain. The fluence-dependent yield of unrepaired deoxyribonucleic acid (DNA) parental-strand breaks following UV irradiation and incubation in minimal growth medium was similar in both strains. However, the fluence-dependent yield of unrepaired DNA daughter-strand gaps observed following UV irradiation was 1.8-fold greater in the uvrA polA strain than in the uvrA strain. These results suggest that DNA polymerase I is involved in the filling of at least some daughter-strand gaps during postreplication repair. Also, the uvrA polA strain was sensitized by a post-UV treatment with chloramphenicol (CAP) to a similar extent as was the uvrA strain, indicating that DNA polymerase I is not involved in the CAP-inhibitable pathway of postreplication repair.