Synthetic wastewater containing Cu(II) in concentrations between 0 and 22 mg L-1 was treated in an activated sludge unit, and chemical oxygen demand (COD), Cu(II), and toxicity removals were investigated. The feed Cu(II)(o) concentrations were varied between 0 and 22 mg L-1 while the feed CODo, hydraulic residence time (HRT) and solids retention time (SRT) were kept constant at 2,000 +/- 100 mg L-1, 10 h and 10 days, respectively. The effluent CODe and Cu(II)(e) concentrations, toxicity, and the sludge volume index (SVI) increased, but the biomass concentration in the aeration tank decreased with the feed Cu(II)(o) content due to Cu(II) toxicity on the organisms. The feed and the effluent toxicities were determined by using the resazurin assay based on dehydrogenase activity. Cu(II) toxicity in the feed and the effluent wastewater increased linearly with the Cu(II) concentrations. Adverse effects of Cu(II) were more pronounced for the feed Cu(II)(o) above 10 mg L-1, resulting in sharp decreases in biomass concentration and percent COD removal. A kinetic model describing the Cu(II) inhibition on COD removal rate was developed and the Cu(II) inhibition constant was found to be 2.85 mg L-1.