This study investigates the biooxidation of a refractory gold concentrate using a mixed culture of acidophilic mesophiles, moderate thermophiles and extreme thermophiles and their effect on the subsequent cyanidation and gold recovery. The experiments with high % solids using mixed mesophiles showed better oxidation potential compared to moderate thermophiles and extreme thermophiles. However, the extreme thermophiles performed better than mesophiles and moderate thermophiles during the biooxidation with <5% solids (w/v). The biooxidized residues obtained from the experiments with extremophiles, gave a maximum gold recovery of 92% after cyanidation. The NaCN consumption during cyanidation of the biooxidized residues obtained from experiments with extremophiles was lower than bioresidues obtained from experiments with mesophiles and moderate thermophiles. The highest consumption of NaCN of 17.5 kg/ton was from the experiment with 10% solids (w/v) using mesophiles. X-ray diffraction studies and chemical analysis of the biooxidized residues confirms the precipitation of jarosites at high redox potential which produces some ferricyanide, thereby explaining the reason for high cyanide consumption during cyanidation. (C) 2010 Elsevier B.V. All rights reserved.