The U.S. Department of Energy has been charged with assessing the suitability of a geologic nuclear waste repository at Yucca Mountain (YM), NV. Microorganisms, both those endogenous to the repository site and those introduced as a result of construction activities, may contribute to the corrosion of metal nuclear waste packaging and thereby decrease their useful lifetime as barrier materials. Evaluation of potential Microbiological Influenced Corrosion (MIC) on candidate waste package materials was undertaken in reactor systems incorporating the primary elements of the repository: YM rock (either non-sterile or presterilized), material coupons, and a continual feed of synthetic YM groundwater. Periodically, both aqueous reactor efflux and material coupons were analyzed for chemical and surfacial characterization. Alloy 22 coupons exposed for a year in reactors containing non-sterile YM rock demonstrated accretion of chrome oxide and silaceous scales, with what appear to be underlying areas of corrosion.