The study of spontaneous andradiation-induced mutations is at present our best approach to the investigation of genetic mechanisms in bacteria. Mutations involving resistance to destructive agents (bacteriophage, l penicillin2) are especially suitable for genetic analysis, since resistant mutants can easily be detectedin bacterial cultures. This preliminary report concernsa mutation in Escherichisa coli leading to resistance to both ultraviolet radiation and x-rays, which was detected by exposing samples from normal cultures to high doses of radiation. Most investigators of the effects of ultraviolet radiation on bacteria have considered the population within a strain to be fundamentally uniform in sensitivity. Most of the differences found seem to depend upon transient