Use of microorganisms for bioleaching processes is commonly limited by low tolerance of extreme culture conditions such as temperature, pH, and pulp density. Adaptation to extreme conditions is the main approach for strain improvement. Traditional methods for breeding strains with tolerance of extreme condition are difficult to apply without genetic tools because of many of such strains are not readily purely cultivated in liquid or agar medium. Here, we developed a method using ultraviolet (UV) irradiation for mutation in a continuous stirred tank reactor (CSTR) for screening to improve the bioleaching performance of a poorly characterized strain of Acidianus brierleyi (A. brierleyi). An improved strain developed with our method exhibited a good tolerance to Cu2+ and a high bioleaching rate of chalcopyrite, which was four-fold better than that of a strain selected by traditional adaptation method. We also designed a countercurrent bioleaching process in stirred tanks in series, and copper recovery in 7-8 days at a 15% chalcopyrite pulp density was more than 90% in a two stage countercurrent reactor. (c) 2006 Elsevier Inc. All rights reserved.