Xylanase A from alkaliphilic Bacillus halodurans C-125 was expressed in Escherichia coli and purified by affinity and anion exchange chromatographies. It exhibited a strong substrate inhibition using xylan as the substrate. Its k(i) value increased with an increase in pH. The effect of pH on the enzyme activity was determined using two aryl-xylobiosides as substrates, and it was found that the enzyme had a flat k(cat)-pH curve in the pH range of 5.8-8.8. This range was different from that obtained with 0.45% xylan as previously reported (Honda, H. et. al., Agric. Biol. Chem., 49, 3165-3169, 1985). The substrate inhibition was presumed to cause the difference. It has been clarified that the use of aryl-xylobiosides as substrates yields more accurate kinetic results than that of xylan.