Three mutants of Trichoderma reesei were grown in solid-state fermentation (SSF) in flasks and in a pan bioreactor. Mutant strain MCG 80 proved to be best at producing an optimal cellulase system using lignocellulosic material (wheat straw [WS]) as substrate. This preparation exhibited a beta-glucosidase activity (beta GA) to FPA (FPA) ratio of about 1.0, which is indicative of a high potential for hydrolysis of cellulose. The yields of cellulase systems and the ratio of beta GA to FPA produced in flasks were comparable to that of the pan bioreactor. The cellulase system of T. reesei MCG 80 having a ratio of beta GA to FPA close to 1.0 gave the most complete (88-95%) hydrolysis of 5% delignified wheat straw (DWS). On the other hand, the cellulase system of cocultures of T. reesei QMY-1 and Aspergillus phoenicis failed to produce high hydrolytic yields in spite of having a very high ratio of beta GA to FPA (3.04). This failure was owing to the fact that coculture contained the relatively poor-quality cellulase system of the dominant organism; A. phoenicis. The resulting fermented WS can be used, as a source of enzyme (unextracted), for hydrolysis of wheat straw, and it gives increased yields of reducing sugars compared to analogous. extracted enzyme preparations. The hydrolytic potential of two commercial enzymes tested were considerably lower than those of the cellulase systems produced on WS. It is evident that a complete cellulase system having a beta GA-to-FPA ratio close to 1.0 and high hydrolytic potential can be produced on lignocellulosic feedstocks in SSF.