Caldicellulosiruptor saccharolyticus has attracted considerable attention by virtue of its ability to degrade various polysaccharide, oligosaccharide and monosaccharide substrates at temperatures above 70 degrees C, while its ability to convert various sugars to hydrogen has led to C. saccharolyticus being selected for the fermentative production of hydrogen. In this study, the utilization of a pure cellulosic substrate and mixed biomasses of plant origin was investigated. Cellulase biosynthesis can be triggered by growing cells on various monomeric carbohydrates, e.g. glucose or fructose. Pretreatment with cellulase-producing Bacilli improves the hydrogen yield, indicating that C. saccharolyticus alone can only partially decompose cellulosic substrates. The hydrogen-producing activity of C. saccharolyticus can be exploited in biogas technologies. With appropriate induction of the polymer-degrading enzymes, C. saccharolyticus may become a prime candidate with which to improve the yield and efficacy of practical hydrogen- and biogas-producing processes.