The effect of 4-chlorophenol (4-cp) on the total treatment efficiency of a high strength synthetic wastewater was investigated and 7 the feasibility of using glucose as a growth substrate for cometabolic transformation of 4-cp by a high density fed-batch reactor evaluated. The experiments demonstrated that 4-cp inhibited the oxidation of the growth substrate, glucose. The total treatment efficiency of 5000 mg l(-1) glucose, in terms of dissolved organic carbon (DOC), decreased from > 99% for a 4-cp free cycle to 66% for an initial 4-cp concentration of 300 mg l(-1). However as the concentration of glucose and the rate of feeding were decreased to 2000-3000 mg; l(-1) and 450 ml h(-1) respectively, complete removal of 300 mg l(-1) 4-cp with a low residual DOC was achieved. Phenol induction prior to inoculation was not a prerequisite to ensure transformation of 4-cp when glucose was the added growth substrate. The results of this work suggest that 4-cp can be degraded by cells grown on glucose as the sole growth substrate and the feeding strategy is the key factor for complete removal of both growth and nongrowth substrates in fed-batch operation. (C) 2002 Elsevier Science Ltd. All rights reserved.