Element-sensitive. bacteria cannot grow because they lack the appropriate detoxifying enzymes. When the toxic element reaches a threshold value in the cytoplasm, the regulatory unit on the plasmid or chromosome somehow switches on the whole genetic sequence of metal resistance. The adverse effects of boron on biological treatment of boron containing wastewater by activated sludge culture were investigated. A synthetic wastewater composed glucose, urea, KH2PO4, MgSO4, KCl, Fe2SO4 and various concentrations of boron (0-5000 mg l(-1)) was treated in an aerobic-biological reactor operating in fed-batch mode. An activated sludge culture obtained from a wastewater treatment plant was used as the seed. Variations of chemical oxygen demand (COD) removal rate and efficiency with boron concentration were determined. Feed H3BO3-B concentration above 2500 mg l(-1) resulted in significant effluent CODe levels at 10 days sludge age (theta(c)) and 9 h hydraulic residence time (theta(H)). A rate expression including boron inhibition effect was proposed and kinetic constants were determined by using the experimental data. (C) 2003 Elsevier Ltd. All rights reserved.