Bioleaching has gained increased interest as an alternative for processing zinc sulfide ores without the generation of SO(2). The bioleaching of sphalerite with mesophile microorganisms at 1 % pulp density has been studied. Batch experiments were carried out at 34 degrees C and 200rpm. The effects of pH, concentration of Fe(II), as well as the presence of Fe(III) in the zinc extraction were assessed. Fast zinc dissolution can be achieved working with Acidithiobacillus sp. The best pH for bioleaching is in the 1.75-2.00 range and the presence of Fe(III) has a strong influence in zinc extraction, increasing the rate of dissolution and does not adversely affect the growth of the Acidithiobacillus population. (c) 2004 Elsevier Ltd. All rights reserved.