The extracellular beta-xylosidase (EC 3.2.1.37) excreted by the thermophilic eubacterium Rhodothermus marinus when grown on xylan has been investigated. The enzyme has been partially purified by ultrafiltration and gel filtration, and some of its characteristics are presented. Rhodothermus marinus grew on xylan with mu max = 0.4 h(-1) and the beta-xylosidase activity was 50 nkat/mL after 24 h in a batch fermentation. The beta-xylosidase activity had a half-life of more than Ih at 90 degrees C and of 14 h at 85 degrees C. At 80 degrees C, 80% of the initial activity remained after 24 h. The initial activity increased with increasing temperature, showing maximal activity at 90 degrees C. The beta-xylosidase had a pH-optimum of 6 and was stable in the range between pH 5 and 9. At pH 10 and 11, 82 and 66%, respectively, of the initial activity remained after 24 h when incubated at 65 degrees C. The molecular weight was estimated to be 169,000 dalton by gelfiltration.