A comparative study on bioleaching with mesophilic and moderate thermophilic bacteria has been performed. The study was carried out by continuous bioleaching with a series of three reactors with a total volume of 20 l. A refractory gold flotation concentrate consisting mainly of pyrite and with a relatively high arsenopyrite content has been used for the study. The correlation between the pyrite and the arsenopyrite oxidation levels and the gold recoveries after cyanidation has been determined. The leaching rates of pyrite in the different experiments as well as the calculated oxygen uptake rates are presented. Anomalies in the gold recoveries between experiments with different types of bacterial are discussed.