This work looks for a better understanding of the biodegradation of xenobiotic azo dyes mediated by yeasts. During a screening program of phenolic acid assimilating capacities it was found that a non-conventional ascomycetous yeast isolate, identified as Candida oleophila, efficiently decolorizes agar plates supplemented with the commercial textile diazo dye Reactive Black 5. Aerobic batch cultures of C. oleophila could completely decolorize up to 200 mg dye l(-1), an ability not yet reported for this yeast species. Moreover, this performance has been achieved in just 24 h of incubation at 26 degrees C in the presence of as little as 5 g glucose l(-1) and without visible signs of dye adsorption to yeast cells. It was found that decolorization occurs during the exponential growth phase and neither laccase nor manganese-dependent peroxidase activities were detected in the culture medium. As far as the decolorization mechanism is concerned, our results indirectly suggest the involvement of an azoreductase-like activity in azo bonds cleavage. (c) 2005 Elsevier Inc. All rights reserved.