The effects of Cr(VI) and Cr(III) species on the activated sludge growth rate have been assessed for a batch growth system, for a range of chromium concentration between 0 and 320 mg l(-1). Cr(VI) was found to stimulate microbial growth for concentrations up to about 25 mg l(-1), exhibiting maximum growth stimulation at 10 mg l(-1), whilst the lethal dose was found to be between 80 and 160 mg l(-1). On the other hand, Cr(111) was also found to stimulate microbial growth for concentrations up to about 15 mg l(-1), (with a maximum stimulation concentration at 10 mg l(-1)), whilst the lethal dose was found to lie between 160 and 320 mg l(-1). The results indicate that Cr(VI) is more toxic to biomass at relatively high concentrations (higher than 70 mg l(-1)) whilst it has a more pronounced growth stimulation effect at relatively smaller concentrations (less than 25 mg l(-1)), compared with Cr(III). (c) 2005 Elsevier B.V. All rights reserved.