Hexavalent chromium, which is frequently encountered in the tannery effluents and in the electroplating wastes, is normally considered toxic to the activated sludge culture. Often a physico-chemical pretreatment prior to biological treatment is considered necessary. However, in this study it has been shown that an acclimatized activated sludge culture is fully capable of coping with the Cr(VI)-containing wastes and moreover the culture was significantly stimulated by up to 25 mg l-1 Cr(VI). None of the tested concentrations of this metal were inhibitory to the mixed culture under investigation. In fact, all of the concentrations tested, 1, 5, 10 and 25 mg l-1 Cr(VI), significantly stimulated the maximum growth rate, mu-m, of the culture. Furthermore, the biomass yield of the activated sludge culture markedly increased in the presence of Cr(VI) while the effect on the biomass decay or maintenance constants was variable.