The anaerobic rumen fungus Piromonas communis, when cultured on cotton fibre as the carbon source, produces an extracellular cellulase that is capable of solubilizing "crystalline" hydrogen-bond-ordered cellulose, in the form of the cotton fibre, at a rate that is greater than that of any other cellulases reported in the literature hitherto. The cell-free culture fluid is also very rich in xylan-degrading enzymes. The activity towards crystalline cellulose resides in a high-molecular-mass (approximately 700-1000 kDa) component (so-called crystalline-cellulose-solubilizing component, CCSC) that comprises endo (1 --> 4)-beta-D-glucanase (carboxymethylcellulase), beta-D-glucosidase and another enzyme that appears to be important for the breakdown of hydrogen-bond-ordered cellulose. The CCSC is associated with only a small amount of the endo(1 --> 4)-beta-D-glucanase (1.9%), beta-D-glucosidase (0.7%) and protein (0.5%) found in the crude cell-free cellulase preparation. The CCSC, unlike the bulk of the endo(1 --> 4)-beta-D-glucanase and beta-D-glucosidase, is very strongly absorbed on the microcrystalline cellulose, Avicel.