Immediate environmental conditions expose the fungi organisms to a multitude of stresses in an in situ bio-leaching process. The most significant abiotic stress is from heavy metals which are dissolved by the metabolic products of the organism. Heavy metal stress can stimulate changes in the fungi metabolism which could inactivate its cellular functions critical in maintaining bio-acid production and leaching of minerals. Gradual acclimatization of the organism to increasing concentration of single metals forms the main method of adapting strains for this process. However in a bioleaching process, several metals are dissolved which exposes the organism to various levels of cation toxicity. In this study Aspergillus foetidus was trained using multi metal (Al, Co, Cr, Cu, Fe, Mg, Mn, Ni and Zn) environment to mimic a leaching system. The effect of the metal stress on the fungi metabolism was examined. Bioleaching of weathered saprolite ore using the heavy metal tolerant organism was assessed. (C) 2006 Elsevier Ltd. All rights reserved.