Abstract The resynthesis step of the excision repair pathway has been examined in Escherichia coli K12 strains isogenic except for mutations in the uvrD cistron. Strains mutant at the uvrD3, uvrD101, uvrE156, and recL152 loci exhibited slight but distinct differences in their response to ultrviolet radiation. The repair capacity of the uvrD101 mutant was given special attention. Repair resynthesis in this mutant was saturated at fluences greater than about 20 J/m 2. Isopycnic analysis of repaired deoxyribonucleic acid from this strain revealed a two-fold increase over it wild-type counterpart in the amount of repair replication performed after a dose of 15 J/m 2. Sedimentation velocity analysis of DNA after selective photolysis of bromouracil-containing repaired regions showed that the uvrD101 mutation exerted its primary effect on the long-patch component of excision repair. The uvrD101 mutant performed long-patch repair at a larger number of sites than the number repaired by this mode in the wild-type strain; these patches were more extensive in length than the long-patch component in wild-type.