Abstract: A xylanase obtained from Rhodothermus marinus, the method of obtaining it and a process for its use are disclosed. The xylanase has an optimum temperature of from 85.degree. to 100.degree. C., a relative activity of more than 50% in the interval of from pH 5 tp pH 8 after incubation for 5 minutes at 65.degree. C., a relative temperature stability at 80.degree. C. of more than 80% after incubation at pH 7 for 3 hours, a pH optimum of about 6 and an isoelectric point in the range of from 3 to 7. The xylanase can be obtained from the strains ATCC 43812, ATCC 43813 or a mutant thereof. The process for obtaining the xylanase involves cultivation of a xylanase-producing strain of Rhodothermus marinus in a suitable nutrient medium containing carbon and nitrogen sources and inorganic salts followed by recovery of the xylanase. Also disclosed is a process for the treatment of lignocellulosic pulp with the xylanase.