Subsurface bacterial isolates obtained through the DOE Subsurface Science Program were tested for resistance to UV light, gamma radiation and H₂0₂. Some deep subsurface bacteria were resistant to UV light, demonstrating ≥1.0% survival at fluences which resulted in a 0.0001% survival level of E. coli B. The percentage of UV resistant aerobic subsurface bacteria and surface soil bacteria were similar; 30.8% and 25.8% respectively. All of the microaerophilic subsurface isolates were UV sensitive as defined in this work; however, subsurface isolates demonstrated UV resistance levels similar to reference bacterial strains of the same Gram reaction. These results were not in agreement with the hypothesis that the resistance of an organism to UV is correlated with the amount of solar radiation in its natural habitat. Evidence for photoreactivation and the presence of an SOS-like mechanism was also detected in subsurface bacteria. The presence of UV resistance and photoreactivation in subsurface bacteria that have been shielded from solar radiation for millions of years may point to a limited rate of evolution in the deep subsurface environment. In subsurface bacteria, there was a relatedness between UV resistance and resistance to gamma radiation and H₂0₂ UV-resistant aerobic subsurface isolates were also gamma and H₂0₂- resistant compared to the microaerophilic isolates tested. Due to the similarities of bacterial responses to UV, H₂0₂ , and gamma radiation, either UV or H₂0₂ may be utilized to model the effects of ionizing radiation on bacterial cultures used for the bioremediation of organic and radioactive waste-containing environments. / Master of Science