Abstract: At many abandoned mine sites in the Western U.S., conventional treatment of AMD is not feasible due to the of lack of power and limited site accessibility. Therefore, three bioreactors were built at an abandoned mine site in Montana to demonstrate feasibility of treating AMD using sulphate reducing bacteria (SRB) in a passive water treatment train. The SRB are capable of increasing the pH and reducing the load of dissolved metals in the effluent. The reactors, constructed in the Fall of 1998, were designed to evaluate the SRB technology applied under different environmental conditions. Each bioreactor was designed with mechanisms to enable simulation of seasonal dry and wet climatic conditions. Two bioreactors were placed in trenches and one was constructed above the ground to investigate impact of seasonal freezing and thawing on SRB activity. Two bioreactors contain a passive pretreatment section to increase pH of water before the AMD enters the bioreactor chamber.