Abstract: Thiobacillus ferrooxidans and Thiobacillus thiooxidans play major roles in bacterial leaching of metals from ores and tailings through their oxidative action on ferrous iron (Fe2+) and sulfide or sulfur (S0). We have studied the effects of various inhibitors such as cyanide, azide and anions (phosphate, nitrate, chloride) on Fe2+ or S0 oxidation by resting cells, on Fe2+ or S0 as an energy source for growth and finally on leaching of Fe or Zn from a sample containing pyrite (FeS2) and sphalerite (ZnS). All of these inhibitors inhibited Fe2+ oxidation more strongly than S0 oxidation and generally stopped the growth on Fe2+ at lower concentrations than on S0. All three anions inhibited the leaching of Fe more strongly than Zn leaching, but potassium phosphate was the most selective. In the leaching experiments with T. ferrooxidans and T.