Xplanases (EC 3.2.1.8) catalyze the hydrolysis of xylan, the major constituent of hemicellulose. The use of these enzymes could greatly improve the overall economics of processing lignocellulosic materials for the generation of liquid fuels and chemicals. The hyperthermophilic archaeon Pyrodictium abyssi, which was originally isolated from marine hot abyssal sites, grows optimally at 97 degreesC and is a prospective source of highly thermostable xylanase. Its endoxylanase was shown to be highly thermostable (over 100 min at 105 degreesC) and active even at 110 degreesC. The growth of the deep-sea archaeon P. abyssi was investigated using different culture techniques. Among the carbohydrates used, beech wood xylan, birch wood glucuronoxylan and the arabinoxylan from oats pelt appeared to be good inducers for endoxylanase and beta -xylosidase production. The highest production of arabino-furanosidase, however, was detected in the cell extracts after growth on xylose and pyruvate, indicating that the intermediate of the tricarboxylic acid cycle acted as a nonrepressing carbon source for the production of this enzyme. Electron microscopic studies did not show a significant difference in the cell surface (e.g., xylanosomes) when P. abyssi cells were grown on different carbohydrates. The main kinetic parameters of the organism have been determined. The cell yield was shown to be very low owing to incomplete substrate utilization, but a very high maximal specific growth rate was determined (mu (max) = 0.0195) at 90 degreesC and pH 6.0. We also give information onthe problems that arise during the fermentation of this hyperthermophilic archaeon at elevated temperatures.