Geotrichum sp. was selected from among several fungi because of its ability to transform three reactive azo dyes (Reactive Black 5, Reactive Red 158 and Reactive Yellow 27) used in industry. None of the white rot fungi tested (Bjerkandera adusta, Ganoderma sp., Irpex lacteus, Phanerochaete magnoliae, Rigidoporus sp., and Trametes versicolor) were able to transform the yellow and red dyes, although Remazol Brilliant Blue R was readily transformed. When each dye was cultivated together with Geotrichum sp., the fungus transformed the black dye rapidly whereas the other two dyes required twice as long. When 20-day old cultures were spiked with successive amounts (200 ppm) of dyes, the time for total transformation was reduced to about 5 days for all three of the reactive azo dyes. It is likely that the lignolytic enzymes Mn peroxidase, Mn-independent peroxidase and laccase were involved in the transformation of the black dye. For the yellow and red dyes we suggest that additional enzymes or factors may be involved. The sustained ability of Geotrichum sp. to transform large amounts of dyes (total of 800 ppm after successive additions) suggests that it may have potential application in the decoloration of textile wastewater.