Certain metals are essential to biological actions. However all metals, whether essential or inessential, will tend to show toxicity at certain levels. Previous attempts to biologically leach nickel oxide ores are hampered by poor tolerance of the strains to heavy metals, which suggest the need for new strains with resistance to high concentrations of heavy metals. This study investigates the development of training strategies, which is attempting to address this issue. The heavy tolerance development of Aspergillus niger, Penicillium simplicissimum, Aspergillus foetidus and Aspergillus carbonarius strains in the presence of Ni, Co, Fe, Mg and Mn was studied up to concentrations of 2000 ppm. The adaptive behaviour was measured from the rates of initial growth and death rate of the strains with time. The effect of type of metal, the metal concentration and the type of strains on this adaptive behaviour was investigated. The results indicate that these initial adaptive behaviours are reflections of the strains tolerance development with increasing metal concentration. A high growth followed by a weaker death rate appear to lead to tolerance development of strains in increasing metal concentration often exceeding the control. Whereas a strong growth followed by an equivalently high death rate appear to lead to poorer tolerance. The relative toxicity of the various metals to the strains was used to select a strain that would be most suitable in leaching nickel laterite ores. (C) 2003 Elsevier Science Ltd. All rights reserved.