The effect of two flotation reagents: isopropyl- and ethyl-xanthates and two solvent extraction reagents (SX): Lix 984 and Lix 860IC on the bioleaching of a copper concentrate with Sulfolobus metallicus was analyzed. The experiments were performed in 250 nil shake flasks with 100 ml basal medium with an initial pH of 1.6, inoculated with a pure culture of S. metallicus and 0.5% copper concentrate (38% of Cu) at 70 degrees C. In the bioleaching flasks 0, 50 or 500 mu g/mL of flotation reagent and 0, 10 or 100 mu g/mL of SX reagents was added. All organic reagents caused a strong inhibition of ferrous iron oxidation and growth of planktonic cells of S. metallicus. However, bioleaching in presence of 50 mu g/mL of isopropyl-xanthate or 10 mu g/mL of Lix 984 reached copper dissolutions of 70% and 72% respectively after 200 hours, compared with 98% in the process without organic reagents. These results suggest that isopropyl-xanthate and Lix 984 at the lowest concentrations used in this study would not affect the mechanism of bioleaching by contact of S. metallicus on the sulphide, however it strongly affected the indirect mechanism. Ethyl-xanthates and Lix 860IC, beginning with 50 mu g/mL and 10 mu g/mL respectively, generated a total inhibitory effect on the oxidation activity of S. metallicus on the sulphides.