The work described here concerns the study of the tolerance limits of the acidophilic bacterium, Acidithiobacillus ferrooxidans, in the presence of a range of heavy metal concentrations. With the aim of reducing the inhibitory effect that organic matter has on ferrous iron oxidation by A. ferrooxidans, a heterotrophic bacterium, Acidiphilium, sp., was added to media supplemented with organic matter. The maximum tolerated concentrations (MTC) were measured. At. ferrooxidans tolerates high levels of the metals tested: 0.4 g Cr(III)/l, 10 g Cu(II)/l, 10 g Cd(II)/l, 30 g Zn(II)/l and 30 g Ni(II)/l. The addition of heterotrophic bacteria, Acidiphilium, changes the tolerance limits in different ways depending on the metal: 0.4 g Cr(III)/l, 4 g Cu(II)/l, 10 g Cd(II)/l, 40 g Zn(II)/l and 15 g Ni(II)/l. (c) 2005 Elsevier Ltd. All rights reserved.