A mutant of rad-8, originally isolated on the basis of its hypersensitivity to ultraviolet radiation, is also hypersensitive to oxygen and methyl viologen, a superoxide-anion generator. Oxygen also retarded development and reduced fecundity in a concentration-dependent fashion in rad-8 but not in wild type. In addition, the mean life span of rad-8 (but not wild type) was progressively shortened when animals incubated in increasing oxygen concentrations. This cross hypersensitivity to both UV radiation and free radicals provides further evidence that DNA damage may be important in the aging process.