An extreme thermophilic, iron-sulphur oxidising bacterial culture was isolated and adapted to tolerate high metal and solids concentrations at 70 degrees C. Following isolation and adaptation, the culture was used in a continuous multi-stage pilot plant employing standard mechanically agitated and aerated tanks, for the bioleaching of a mixed secondary copper sulphide-chalcopyrite concentrate. The culture exhibited stable leach performance over the period of pilot plant operation and overall copper extractions of higher than 97% were maintained. It was shown, however, that solids concentration had a significant effect on the copper bioleach kinetics. Increased solids concentrations resulted in a drop in redox potential and copper extractions in the first stage reactor. Batch chemical ferric leaches, performed at controlled redox potentials, indicated that reduced residence times may be possible in bioleach processes, using thermophiles, if high redox potential levels can be maintained.