The purpose of this study was to evaluate the toxic effects of Cu(II) on a chemostat containing activated sludge. Different concentrations of Cu(II) (0.5, 1.0, 5.0, and 10.0 mgL-1) were maintained in a laboratory-scale completely mixed activated sludge unit, without recycle, treating simulated wastewater. The feed solution contained 650 mgL-1 protein (corresponding to 1300 mgL-1 COD) as a source of carbon. Experimental results indicated that the treatment efficiency was not adversely effected by the presence of Cu(II) up to the concentration of 10.0 mgL-1. Moreover, some stimulatory effects of Cu(II) on the biokinetic parameters were observed.