Cultivation of the extreme thermophilic anaerobic bacterium Thermotoga thermarum at 77 degrees C on xylan was accompanied by the formation of heat-stable endoxylanase (136 U/l), beta-xylosidase (44 U/l) and alpha-arabinofuranosidase (10 U/l). These enzymes were mainly associated with the cells and could not be released by detergent treatment (0.1-1.0 mM 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulphonate (CHAPS). Endoxylanases with a molecular weight of 40, 83 and 100 kDa were induced when xylan or xylose were used as substrates for growth. In the presence of other sugars like glucose, maltose, arabinose or starch, low concentrations of the low-molecular-weight endoxylanase (40 kDa) was detected. Xylose was found to be the best substrate for the induction of beta-xylosidase and alpha-arabinofuranosidase but not for growth. Cultivation of T. thermarum in a dialysis batch fermenter resulted in a significant increase in cell concentration and enzyme level. A total cell count of 1.3 x 10(9) cells/ml and 202 U/l of endoxylanase were measured when partially soluble birchwood xylan was used as the carbon source. The use of insoluble beechwood xylan as the substrate caused the elevation of the maximal cell concentration and enzyme level up to 2.0 x 10(9) cells/ml and 540 U/l, respectively.