Aeromonas caviae W-61, which was isolated from water samples at the Faculty of Agriculture, Tohoku University, produced beta-1, 4-xylanase (1,4-beta-d-xylan xylanohydrolase; EC 3.2.1.8) extracellularly. The xylanase was purified to homogeneity by using DEAE-Sephadex A-50, CM-Sephadex C-50, and Sephadex G-100 column chromatographies. The molecular weight of the purified enzyme was estimated to be 22,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric point of the enzyme was 9.2. The optimal pH and temperature for the activity of the enzyme were 7.0 and 55 degrees C, respectively. The enzyme was stable at pH 7.0 at temperatures of up to 50 degrees C. As enzymatic products, various xylo-oligosaccharides such as xylobiose, xylotriose, xylotetraose, and xylopentaose were formed, and only a small amount of xylose was detected. The purified enzyme did not hydrolyze starch, cellulose, carboxymethylcellulose, or beta-1, 3-xylan.