Abstract The thin-layer leaching process originally conceived and developed for leaching oxide ores has been successfully adapted to bacterial leaching of mixed and secondary sulphide ores. The process is currently being applied at the Socicdad Minera Pudahuel Lo Aguirre Plant. About 3000 ton of ore per day are being processed to produce 14000 ton of high-grade copper cathodes per year, in a closed circuit integrated with SX-EW. Changes in the soluble copper grade of the ore from about 1.8C4 to (I.6% have occurred during the last years, which have been compensated by an equivalent increase in the insoluble copper grade. In addition, ore from satellite ore bodies has resulted in acid consumption variations ranging from 611 to 120 kg H 2SO 4 per ton of ore. The main sulphide mineralogical species are chatcocite and bornite, with small amounts of chalcopyrite and covellite. An intensive research program in columns and large-scale heaps has been carried out to define the operating conditions which assure adequate bacterial growth and bacterial activity towards the sulphides. Agglomerated ores with 1.7 2.5% Cur and (I.3-0.6%: Cus, with the insoluble copper mainly present as chalcocite bornite, were leached at a flow rate of 0.2 1 min 1 mu 2 with a SX-Raffinate solution containing (in g -1) 5–10 H 2SO 4, 2–4 Fe T, 1–3 Fe +3, 0.5 Cu, as well as impurities resulting from a closed circuit operation. Copper recoveries of 75 85% Cu T were obtained after 180–250 days of total leaching time, depending on the copper grade, the mineralogical composition, and the acid consumption of the ore. Important bacterial activity was detected. About 10 3−10 5 bacteria ml −1 were measured in effluent solutions. Iron oxidation rates of 7–100μg Fe 2+ h −1 g −1, measured from respirometric tests on agglomerated ore, suggest that an adsorbed biomass of about 10 7-10 8 bacteria g −1 must also be present. Further applications of the bacterial thin-layer leaching process to Cerro Colorado and Qucbrada Blanca ores in North Chile are being studied.