A cellulase from the thermophile, Thermotoga maritima, hydrolyzed oligosaccharide substrates by an exoglucanase mode of action but acted as an endoglucanase to rapidly reduce the Viscosity of the soluble polysaccharides carboxymethylcellulose and barley beta-glucan. The V-max for hydrolysis of the substrate, p-nitrophenyl beta-D-cellobioside, was 42 mu mol min(-1) (mg protein)(-1), while that for barley beta-glucan was 637. The enzyme had little activity on crystalline cellulose.