A pilot scale investigation of the treatment of bleached kraft mill wastewater with selector-assisted activated sludge (AS) and aerated stabilization basin (ASB) systems was conducted at a northern Ontario mill site over an 8-month period. The objective of this work was to demonstrate the potential of these systems to remove acute and chronic toxicity as well as a wide range of toxic contaminants. The AS system was operated at 1 day nominal HRT, 25-30 days SRT and 30oC. The ASB performance was evaluated at 10 and 20oC, and at nominal HRTs of 5 and 10 days. Treated effluents from both systems were found to be non-toxic in both acute and chronic toxicity tests. Low levels of AOX, total chlorophenolics, toxicity equivalents (TEQ-PCP), and total resin and fatty acids, were achieved by both AS and ASB systems except for the ASB when operating at the lower temperature. Eleven- to 12-fold reductions in hepatic MFO enzyme activity, as compared to the influent wastewaters, were measured for the ASB effluent when operated at the higher temperature, whereas 8- to 17-fold reductions occurred in the AS effluent.