The rumen anaerobic fungus Orpinomyces joyonii A₄ was grown in a complex medium with glucose, xylose, microcrystalline cellulose or waste paper. The fungus produced significantly (P<0.05) more formate, acetate and ethanol, but less lactate from xylose than from glucose. Protein content and yields of dry matter and protein in cells grown on glucose and xylose were not significantly different. Fungal cells contained 64.8% of crude protein, but only 34.9-35.2% of true protein determined according to Lowry. In dual-substrate fermentation both sugars were utilized simultaneously. O. joyonii A₄ efficiently metabolized microcrystalline cellulose and milled office paper (up to 41.1 and 38.0 mmol of fermentation end-products per l, respectively). Its growth on newspaper was poor (13.4 mmol of fermentation end-products per l). Glucose accumulated in fungal cultures with cellulose (2.55 g/l) and office paper (0.60 g/l) at the end of the incubation.