Immobilized activated sludge was applied to the treatment of organic wastewater discharged from a semiconductor plant. To enhance the treatment efficiency, the growth characteristics of heterotrophic bacteria and the effect of the BOD loading rate in continuous wastewater treatment by the immobilized activated sludge were investigated. The activated sludge was entrapped in polyethylene glycol (PEG) prepolymer pellets. Wastewater with a BOD of 150-200 mg/l was continuously treated in a 200-l capacity prototype reactor. The effluent BOD concentration after treatment was 10 mg/l or less at a loading rate of a 5.21 kg-BOD/m(3) d, representing BOD removal of 95-97%. The layer of heterotrophic bacteria in a pellet was 300 mu m in depth, which was thicker than that of immobilized nitrifying bacteria fed on domestic wastewater.