Three substrates, sago fibre, rice straw and sawdust supplemented with either palm kernel cake, rice bran, sodium nitrate or urea were fermented by the white-rot fungus, Pleurotus sajor-caju, for 0 (control), 10 or 25 days at 25°C in the dark. The rate of supplementation with palm kernel cake and rice bran was 200 g supplement/kg substrate and the rate with sodium nitrate and urea was 100 g supplement/kg substrate. After fermentation the spent waste was analysed for total ash (TA), neutral detergent fibre (NDF), crude protein (CP) and in vitro dry matter digestibility (IVD). All substrate-supplement combinations except with urea, promoted fungal growth. The characteristics of the spent waste of the three substrates after 25 days were different indicating fungus-substrate specificity during fermentation. With sago fibre, primary metabolism of the soluble carbohydrates was not followed by secondary metabolism of the structural carbohydrates. This was reflected in an increase in TA and NDF. With sago fibre, a depressed IVD was associated with an increase in CP reflecting antagonism between these two processes. With sawdust, primary and secondary metabolism of the carbohydrates was evident with the sodium nitrate supplement. With rice straw, although there was also a loss of organic matter, secondary metabolism of the structural carbohydrates occurred with the rice bran and sodium nitrate supplements. Of the three substrates, rice straw supplemented with either palm kernel cake or rice bran yielded a spent waste suitable for animal feeding, with an IVD of 63.3% and composed of 20.0% TA, 11.2% CP and 55.2% NDF.