Thiobacillus ferrooxidans tolerates Cr3+ up to 75 mM during growth on ferrous sulphate without modification of its bacterial activity (53 mu g protein mM(-1) Fe2+ oxidised). At pH 1.4 and in the presence of Cr2O3, a maximal uptake capacity of 509 mg.g(-1) dry weight was obtained with biomass harvested in the middle of the exponential growth phase. Cr6+ uptake by T. ferrooxidans resulted in the precipitation of a chromium-rich compound on the surface bacterial cells, as observed by transmission electron microscopy.