A fermentation process using hemicellulose-derived carbohydrates for the production of cellulase mixtures with a high proportion of cellulases relative to hemicellulases is provided. The cellulases produced by the process of the invention are further characterized by high specific productivity. The resulting cellulase mixtures comprise at least two times more cellulase than hemicellulase and are useful for the hydrolysis of cellulosic substrated, particularly, pretreated lignocellulosic substrate.