A pilot scale investigation of the selector-assisted activated sludge treatment of bleached kraft mill effluent was conducted at a northern Ontario mill site over an 8-month period. The objective of this work was to demonstrate the potential of this systemic to remove acute and chronic toxicity as well as a wide range of toxic contaminants. The activated sludge system was operated at 1 day HRT, 25-30 days SRT and 30° C. Treated effluents passed all acute and chronic toxicity tests as measured by Rainbow trout LC50, Microtox and Ceriodaphnia LC50 and IC25 tests. A high level of effluent quality was achieved with low concentrations of AOX (4-13 mg/L), total chlorophenolics (0.3-32 mg/L), toxicity equivalents (TEQ-PCP) (0.4-5.0 mg/L), total resin and fatty acids (0-4 mg/L), BOD (4-12 mg/L) and soluble COD (142-274 mg/L) being recorded over the whole period of investigation. An 8 to 17 fold reduction in hepatic MFO enzyme activity was measured in the treated effluents over the influent wastewaters.