The bioleaching rate of pyrite (FeS2) by the acidophilic thermophile Acidianus brierleyi was studied at 65 degrees C and pH 1.5 with leach solutions supplemented with yeast extract. In the absence of yeast extract supplementation, A. brierleyi could grow autotrophically on pyrite, and the leaching percentage of pyrite particles (25-44 mu m) reached 25% for 7 d. The bacterial growth and consequent pyrite oxidation were enhanced by the addition of yeast extract between 0.005 and 0.25% w/v : the pyrite particles were completely solubilized within 6 d. The bioleaching rate was enhanced by a factor of 1.5 when the yeast extract concentration was changed from 0.005 to 0.05% w/v. However, there was only a slight effect on the leaching rate at the yeast extract concentrations of 0.05 to 0.25% w/v, suggesting that the organic supplement level was in large excess in the pyrite bioleaching.