Three induced mutation methods by ultraviolet ray, microwave and multiplex factors were compared to improve the bioleaching activity of Acidiothiobacillus caldus in processing sphalerite. The experimental results showed that the multiplex method with a mean increased growth rate of 101/,, by ultraviolet ray and microwave was better than that with single UV-induced (7.3%) or microwave-induced (5%) method. Moreover, there was a close relationship between the sulfate concentration and the bioleaching rate of sphalerite. The improvement of bioleaching rate by induced mutant strains was attributed to their faster growth rates and stronger sulfuric acid producing capabilities compared with that of wild strains. Further, a continuous multiplex induced mutation and low-pH selection were successfully applied in the selection of superior leaching strains. (c) 2007 Elsevier Ltd. All rights reserved.