Trichoderma reesei LM-UC4 and its mutant LM-UC4E1 were co-cultured with Aspergillus phoenicis QM329 for cellulase production on bagasse by mixed culture solid substrate fermentation. A mutual synergism was observed between the parent Trichoderma strain and the Aspergillus, resulting in enhanced combined biomass production and corresponding increased in cellulase, endoglucanase and β-glucosidase activities. Such synergism was absent with the mutant Trichoderma strain suggesting that in the hypermutation the ability for cooperative interaction with other microbes was lost.