Natural transformation was used to help define a collection of ionizing radiation-sensitive strains of Deinococcus radiodurans. Three putative rec mutations were identified, as were three pol alleles. Forty of the ionizing radiation-sensitive strains were placed into 16 linkage groups, and evidence obtained indicates that each linkage group consists of a cluster of mutations not more than 1,000 bp apart. In addition, a new class of D. radiodurans mutant was described that, although radioresistant, appears to recover from ionizing radiation-induced DNA damage slowly relative to other strains of D. radiodurans.