A fungus, Geotrichum candidum Dec 1, newly isolated from soil as a dye-decolorizing microorganism, decolorized 18 kinds of reactive, acidic and dispersive dyes and 3 model compounds on a solid medium, showing a broad spectrum of decolorization. Except for dispersive dyes, all the dyes used on the solid medium were also decolorized even in a liquid medium, although the decolorizing rates varied depending on the dye structure. By repeated addition of one dye, Reactive blue 5, about 12 g/l of the dye was degraded without significant decline of activity, showing the resistant property of Dec 1 to a high concentration of the dye. An energy source and oxygen were essential for the expression of decolorizing activity; the optimal temperature was 30 degrees C. A crude extracellular enzyme solution, in which the decolorizing activity was more than 100 times that of the Dec 1 culture broth, showed peroxidase activity, indicating that some peroxidases are responsible for dye-decolorization.