The genes involved in the high radiation resistance of mutant R68 of Salmonella typhimurium LT2 were mapped by conjugation. It was observed that the high radiation resistance involved genes localized in two regions of the chromosome, which have been designated as garA and garB for high gamma resistance. The garA gene mapped near gal and uvrB at about 18 map units, and the garB gene mapped near purC at about 49 map units. The resistance of R68 was reduced to the wild-type level by the acquisition of the two wild-type alleles, garA+ and garB+. Recombinants carrying the garA or garB gene repaired single-strand breaks in their DNA faster than did the wild-type strain. However, only those with the garA mutation showed a marked increase in UV irradiation resistance above the wild-type level, whereas those with garB mutation exhibited an increased rate of spontaneous degradation of DNA beyond the level observed in recA cells.