Toxic chlorophenol compounds present in many chemical industry wastewaters cause severe problems in biological wastewater treatment. Para-chlorophenol (4-chlorophenol, 4-CP) containing synthetic wastewater was treated in an activated sludge unit at different sludge ages varying between 3 and 30 days, while the feed chemical oxygen demand (COD) (2,500 mg L-1), 4-CP (500 mg L-1) and hydraulic residence time (HRT, 25 h) were constant in order to investigate the effects of sludge age on COD, 4-CP, and toxicity removals. Biomass concentration in the aeration tank increased from 1,500 to 6,250 mg L-1 with increasing sludge age from 3 to 30 days, resulting in considerable increases in COD, 4-CP, and toxicity removals. Percent COD removal increased from 61 to 93%, while 4-CP and toxicity removals increased from 4 to 99% when the sludge age was raised from 5 to 30 days. The Resazurin method, based on dehydrogenase activity, was used for assessment of the feed and effluent wastewater toxicity. Sludge volume index decreased with increasing sludge age, indicating improved settling characteristics of the sludge at high sludge ages. Specific rates of COD removal increased, but 4-CP removal decreased with increasing reactor 4-CP content, indicating 4-CP inhibition on 4-CP removal for 4-CP concentrations above 200 mg L-1 in the reactor. The system should be operated at a sludge age of 10 days or above in order to obtain more than 90% COD and nearly complete removal of 4-CP and toxicity.