We have observed the enzymatic production of deoxyribonucleic acid (DNA) doublestrand breaks in Escherichia coli K12 after ultraviolet irradiation. Doublestrand breaks appeared in wild-type, polA1, recB21, recA, and exrA strains after incubation in minimal medium. THE UVRA6 strain showed no evidence of double-strand breakage under the same conditions. Our data suggest that uvr+ cells, which are proficient in the incision step of excision repair, accumulate double-strand breaks in their DNA as a result of the excision repair process, i.e., arising from closely matched incisions, excision gaps, or incisions and gaps on opposite strands of the DNA twin helix. Furthermore, strains deficient in excision repair subsequent to the incision step (i.e., polA, rec, exrA) showed more double-strand breaks than the wild type strain. The results raise the possibility that a significant fraction of the lethal events in ultraviolet-irradiated, repair-proficient (uvr+) cell may be enzymatically-induced DNA double-strand breaks.