A thermophilic species, Sulfolobus acidocaldarius, strain BC, has been used for the bioleaching of refractory arsenical pyrite concentrates to enhance Au extraction in cyanidation. The results of laboratory batch tests on two concentrates are presented. In both cases, Au extraction rates were greatly increased from < 11% with only cyanidation to > 90% with the bioleaching pretreatment. The behaviour of arsenopyrite and pyrite is described during the bio-oxidation course. Correlations are determined between the degree of the arsenopyrite/pyrite oxidation and the Au extraction rate.