In one aspect, the invention provides a purified thermostable enzyme derived from the archael bacterium AEPII 1a. In one aspect, the enzyme has a molecular weight of about 60.9 kilodaltons and has a cellulase activity. The enzyme can be produced from native or recombinant host cells, and can be used to aid in the digestion of cellulose. The invention also provides polypeptides having endoglucanase activity having homology to SEQ ID NO:2.