The sorption of Ni2+ by acclimatized activated sludge treating Ni2+ bearing wastewater was investigated using a once-through completely mixed tank reactor. The culture developed from sewage was acclimatized to 85.2 mumole/L Ni2+ influent concentration by stepwise increases, at a low dilution rate 0.11/h. Acclimation was found to enhance the sorptive capacity of the activated sludge. In fact, at all of the intermediate concentrations, percentage Ni2+ adsorbed by the biomass and also the sorptive capacity of the activated sludge drastically increased with an increase in the influent Ni2+ concentration. All influent Ni2+ concentrations were found to significantly stimulate the observed biomass yield of the culture over that observed in the base line. Experimental findings obtained at two other dilution rates; namely, 0.25/h and 0.45/h revealed that dilution rate is a significant operational parameter affecting the Ni2+ sorption characteristics of acclimatized activated sludge microorganisms. Considerable complexation of nickel and organic and inorganic ligands in the wastewater appeared to be responsible for a relatively lower Ni2+ sorption capacity. (C) 2003 Elsevier Science Ltd. All rights reserved.