An aryl beta-xylosidase was purified to homogeneity from an Escherichia coli strain containing a recombinant plasmid carrying a beta-xylosidase (EC 3.2.1.37) gene from the extremely thermophilic anaerobic bacterium isolate Tp8T6.3.3.1 ('Caldocellum saccharolyticum'). It has a pI of 4.3 and shows optimal activity at pH 5.7. The enzyme is highly specific, acting on o- and p-nitrophenyl beta-D-xylopyranosides and minimally on p-nitrophenyl alpha-L-arabinopyranoside. It does not act on xylobiose. The Km for p-nitrophenyl beta-D-xylopyranoside at the optimum pH for activity is 10 mM, and at pH 7.0 is 6.7 mM. Xylose is a competitive inhibitor with Ki 40 mM. Thermal inactivation follows first-order kinetics at 65 and 70 degrees C with t1/2 values of 4.85 h and 40 min respectively. The t1/2 at 70 degrees C is increased 3-fold and 4-fold by the addition of 0.5 mg of BSA/ml and 2 mM-dithiothreitol respectively.