Bioleaching of the copper flue dust emanating from smelters at Sarcheshmeh Copper Complex (Iran) has been studied. At present, the dust is recycled to the smelters which reduces their efficiencies and increases the required energy for their smelting. In this study, a continuous system including a feed tank and two-stage aerated stirred tank bioreactors, each with a worked volume of 2.1 I was established. Mixed mesophile bacteria with different amounts of the dust containing 35% (w/w) of copper mainly in the form of sulfide minerals were used. The effects of pulp densities and residence times on the final copper recovery and redox potential were experienced. Despite of the oxidation of sulfide minerals (especially pyrite) in copper concentrates that produces acid, the process of dust bio-treating was net acid consuming. Lower pulp densities (2% w/v and 4% w/v) resulted in a stable redox potential in both reactors. However, increasing the pulp density to 7% (w/v) created an unstable redox. potential in the first bioreactor. Final copper recoveries were calculated for pulp densities of 2%, 4% and 7% (w/v), were 89.7%, 90.3% and 86.8% with residence times of 2.7, 4 and 6 days, respectively. The promising results indicated that bioleaching is a feasible process that can be applied to copper flue dusts. (C) 2007 Elsevier B.V. All rights reserved.