Bioleaching of pyrite (FeS2) by the thermophilic bacterium Acidianus brierleyi was studied experimentally and theoretically in a stirred-tank reactor operated continuously with respect to both mineral particles and liquid medium. Continuous bioleaching experiments were performed at 65 degrees C and pH 1.5 over the range of dilution rates (0.4-5 d(-1)) and feed pyrite-liquid loading ratios (5 and 20 kg/m(3)). A mathematical model was developed to describe quantitatively the bioleaching process in the continuous-flow mode of operation. The reactor model assumed that the growth rate of adsorbed bacteria on the mineral surface is proportional to the product of the adsorbed-cell concentration and the fracton of adsorption sites by unoccupied by cells, and took into account a distribution of residence time for the mineral particles leaving the reactor. The model developed here and the parameter values determined previously in separate batch tests gave good predictions of the mean leaching fraction and the free cell concentration in the reactor. The present model was also consistent with previous rate data for the bioleaching of zinc sulfide by the mesophilic bacterium Thiobacillus ferrooxidans in a continuous-flow reactor.