Excision repair in ultraviolet-irradiated wild-type Escherichia coli produces a bimodal distribution of repair patch sizes in the DNA. Approximately 99% of the repair events result in short patches of 20-30 nucleotides produced by a constitutive repair system. The remaining 1% result in patches which are at least 1,500 nucleotides in length. This long patch repair is shown to be a damage-inducible process under control of the rec-lex regulatory circuit. The kinetics of the two processes differ; short patch synthesis begins immediately after irradiation and is virtually completed prior to synthesis of the majority of the long patches. Long patch repair synthesis is a linear function of UV dose up to a plateau at 60 J/m2, and hence each long patch event is the consequence of a single UV-induced lesion. Long patch repair does not appear to be necessarily error-prone, since no alteration in repair synthesis occurs as a result of a mutation umuC- which renders cells nonmutable by UV. Evidence is presented suggesting that DNA polymerase I is responsible for both long and short patch synthesis in wild type cells under inducing conditions. In the absence of polymerase I the constitutive patch size averages 80-90 nucleotides, and this distribution is unchanged by induction.