Deinococcus radiodurans R1 and other members of this genus are able to repair and survive extreme DNA damage induced by ionizing radiation and many other DNA-damaging agents. The ability of R1 to repair completely > 100 double-strand breaks in its chromosome without lethality or mutagenesis is recA dependent. However, during the first 1.5 h after irradiation, recA+ and recA cells show similar increases in the average size of chromosomal fragments. In recA+ cells, DNA continues to enlarge to wild-type size within 29 h. However, in recA cells, no DNA repair is observed following the first 1.5 h postirradiation. This recA-independent effect was studied further, using two slightly different Escherichia coli plasmids forming adjacent duplication insertions in the chromosome, providing repetitive sequences suitable for circularization by non-recA-dependent pathways following irradiation. After exposure to 1.75 Mrad (17,500 Gy), circular derivatives of the integration units were detected in both recA+ and recA cells. These DNA circles were formed in the first 1.5 h postirradiation, several hours before the onset of detectable recA-dependent homologous recombination. By comparison, D. radiodurans strains containing the same E. coli plasmids as nonrepetitive direct insertions did not form circular derivatives of the integration units before or after irradiation in recA+ or recA cells. The circular derivatives of the tandemly integrated plasmids were formed before the onset of recA-dependent repair and have structures consistent with the hypothesis that DNA repair occurring immediately postirradiation is by a recA-independent single-strand annealing reaction and may be a preparatory step for further DNA repair in wild-type D. radiodurans.