Trichoderma reesei LM-UC4, the parent strain, and its hypercellulolytic mutant LM-UC4E1 were co-cultured with Aspergillus niger ATCC 10864 in solid substrate fermentation on alkali-treated sugar cane bagasse for cellulolytic enzyme production. Bagasse was supplemented with either soymeal or with ammonium sulfate and urea, and fermented at 80% moisture content and 30 degrees C. Mixed culturing produced better results with the inorganic supplement. The mutant strain was more responsive to mixed culturing than the parent strain when A. niger was the cooperating partner. In a mixed culture of the mutant with inorganic N-source, 10% more biomass, but 63% more cellulase, 85% more endoglucanase and 147% more beta-glucosidase was produced than in single culture. Since co-culturing helped enzyme production more than growth, it appeared that synergistic interactions not directly related to growth were responsible for increased enzyme production. Although soymeal supplementation increased biomass production in the same mixed culture by 30%, it did not increase enzyme production. Mixed culturing is thus beneficial for the economic production of cellulases on nutritionally poor agricultural residues, without the need for supplementation with expensive organic supplements.