Three different reactor configurations (continuous packed-bed bioreactor, fedbatch fluidized-bed bioreactor, and continuous fluidized-bed bioreactor) were designed and tested for decolorization of an azo dye, Orange II, with white rot fungus. Ail the bioreactors showed high and stable decolorization activity in long-term operation. The fedbatch fluidized-bed bioreactor was particularly suitable for Orange II decolorization since it showed very high decolorization efficiency (over 97% color removal of 1,000 mg l(-1) Orange II in one day with immobilized fungal mycelia). Free mycelial pellets and immobilized mycelia were reused continuously in this system for Orange II decolorization for more than I month and 2 months respectively, with continued efficiency.