Effect of 1-25 mg/l of influent Cr(VI) concentrations on true biomass yield (Y-t) and decay coefficient (k(d)) of activated sludge units was investigated in this study. To this purpose, completely mixed, once-through laboratory scale activated sludge units with acclimated cultures were operated. After obtaining dilute-out curves for each Cr(VI) concentration, the values of Y-t and k(d) were determined. The results revealed that presence of Cr(VI) caused stimulatory effects on biomass yield and maintenance up to the Cr(VI) concentration of 25 mg/l. Lower Cr(VI) concentrations exerted higher stimulation of the system. Extended oxidation due to the presence of Cr(VI) and/or increased synthesis of cellular material due to the stimulatory effects of Cr(VI) were thought to be the mechanisms of the observed stimulation. (C) 1999 Elsevier Science Inc. All rights reserved.