An endo-gb-1,4-mannanase cloned from caldocellum saccharolyticum and expressed in Escherichia coli was partially purified. The purification involved heat treatment, anion exchange and gel filtration. The mannanase was only active against mannan, glucomannans and galactoglucomannans and obeyed Michaelis-Menten kinetics on these substrates. The rate and extent of hydrolysis was dependent on the type of substrate. Galactomannans were not as readily depolymerized as the mannan and glucomannans investigated. The glucose content of the glucomannans did not affect the rate of hydrolysis and only slightly affected the extent. The molecular mass of the mannanase was estimated at 39 kDa. The pH and temperature optima were 6.5 and 80° C respectively. The mannanase was very thermostable with a half life of 48 min at 85° C and no loss in activity after 24 h at 70° C.