The effect of mineral particle size on the bioleaching of pyrite by the acidophilic thermophile Sulfolobus metallicus was investigated in a batch bioreactor. Decreasing the particle size from a mean diameter of 202 micron (size fraction: 150-180 micron) to a mean diameter of 42.5 micron (size fraction: 25-45 micron) enhanced the bioleaching rate from 0.05 kg m(-3) h(-1) to 0.098 kg m(-3) h(-1). The particle size distribution of the mineral in this range did not influence the morphology and growth kinetics of the cells. The values of specific growth rate (mu) and yield factor (Y) were 0.018-0.025 h(-1) and 0.67 x 10(-11)-1.45 x 10(11) cells (g iron)(-1), respectively. Decreasing the particle size of the mineral to a mean diameter of 6.40 micron (size fraction <25 micron) adversely influenced the activity of the cells. The presence of fine particles apparently damaged the structure of the cells, resulting in their inability to oxidise pyrite.