In this study, the mineral leaching ability of heterotrophic fungi, through secretion of organic acids, of various laterite ores was studied. Ores subjected to the leaching process included saprolite, weathered saprolite, limonite and nontronite. Strains of Aspergillus and Penicillium were found to be the most efficient organisms. Nickel extraction of up to 36wt%, cobalt of 54wt% and iron of 0.76wt% were achieved using a direct leaching where metals were leached in the same phase where bio-acids were produced. Extractions were compared with chemical leaching, where leaching up to 79.5wt % Ni, 71 wt% Co and 50wt% Fe were leached. The variation in recovery was found to be strongly influenced by the ore mineralogy and the metal loss through electro-sorption properties of the ores. Effective and selective leaching by certain strains for cobalt and nickel, which was not otherwise observed in the chemical leaching process, suggest that microbiological activity, apart from bio-acid production is participating in the leaching process. (C) 2001 Elsevier Science Ltd. All rights reserved.