Bacillus pumilus beta-D-xylosidase, purified by affinity chromatography, seems to be homogeneous, as judged by disc electrophoresis and gel filtration. The absorption coefficient at 280 nm, Ao, 1% 1cm, determined by the dry weight method, is 1.78. The complete amino acid composition is determined. Sedimentation velocity studies show the presence of two components with S20, W values of 10.0 S and 6.6 S. After glutaraldehyde cross-linking two, enzymically active, components, with apparent molecular weights 126 000 and 243 000, can be isolated by preparative sucrose gradient ultracentrifugation. These values are confirmed by analytical disc electrophoresis at different acrylamide concentrations. The subunit molecular weight is 60 000. L-Methionine is the only N-terminal amino acid detectable. The possible presence of both dimeric and tetrameric forms of the beta-D-xylosidase in solution has to be envisaged.