The ability of an extreme thermophile to oxidise a concentrate comprising of chalcopyrite (66%) and pyrite (11%) is described. A batch test at 70 degreesC showed that a copper extraction of > 98% was possible. A series of continuous tests were carried out in a three-stage pilot plant. employing standard-design mechanically agitated and aerated tanks. The effect of residence time, feed grind and the mass transfer supply of oxygen and carbon dioxide on bioleach performance were evaluated. The results showed that overall Cu extractions of 95% could be obtained. However, compared to mesophilic and moderately thermophilic bioleaching bacteria, the extreme thermophiles appeared to be more sensitive to the solids concentration employed, which may also be related to the particle size of the feed solids. Levels of oxygen consumption approaching the maximum rates currently being employed in commercial-scale bioleach tanks treating refractory gold pyritic concentrates could be achieved. To maintain these high oxidation rates, it was important to ensure that the supply rate of oxygen and carbon dioxide to the reactors was sufficient. The high copper extractions obtained in these tests showed that a process treating chalcopyrite concentrates using extreme thermophiles has the potential for further development and assessment for commercial applications.