The effects of adding cellobiose on the transformation of vanillic acid to vanillin by two strains of Pycnoporus cinnabarinus MUCL39532 and MUCL38467 were studied. When maltose was used as the carbon source in the culture medium, very high levels of methoxyhydroquinone were formed from vanillic acid. When cellobiose was used as the carbon source and/or added to the culture medium of P. cinnabarinus strains on day 3 just before vanillic acid was added, it channelled the vanillic acid metabolism via the reductive route leading to vanillin. Adding 3.5 g l(-1) cellobiose to 3-day-old maltose cultures of P. cinnabarinus MUCL39532 and 2.5 g l(-1) cellobiose to 3-day-old cellobiose cultures of P. cinnabarinus MUCL38467, yielded 510 mg l(-1) and 560 mg l(-1) vanillin with a molar yield of 50.2% and 51.7% respectively. Cellobiose may either have acted as an easily metabolizable carbon source, required for the reductive pathway to occur, or as an inducer of cellobiose:quinone oxidoreductase, which is known to inhibit vanillic acid decarboxylation.