A white rot fungus, Coriolus hirsutus, exhibited a strong ability to decolorize melanoidin in cultures not supplemented with nitrogenous nutrients. Addition of peptone to the cultures lowered the ability of, the fungus to decolorize melanoidin, but that of inorganic nitrogens (Ns), ammonium and nitrate did not bring about any marked reduction in the ability. These results suggest an inhibitory effect of organic N on melanoidin decolorization. Therefore, for enhancing the decolorization of melanoidin in wastewaters by the fungus, activated sludge pretreatment of the wastewaters was expected to be effective, i.e., activated sludge is capable of converting available organic N into inorganic N. To confirm this, waste sludge heat treatment liquor (HTL), wastewater from a sewage treatment plant, was pretreated with activated sludge. In practice, pretreatment of HTL under appropriate conditions accelerated the fungal decolorization of HTL. In the pretreated HTL, the fungus was shown to produce a high level of manganese-independent peroxidase (MIP). Addition of Mn(II) to the pretreated HTL caused a further increase in the decolorization efficiency of the fungus and a marked increase in the manganese peroxidase (MnP) activity, Consequently, the increases in MIP and MnP activities were considered to play an important role in the enhanced ability of C. hirsutus to decolorize HTL.