An anaerobic, extremely thermophilic, and cellulose- and xylan-degrading bacterium F32 was isolated from biocompost. Sequence analysis of the 16S rRNA gene of this strain showed that it was closely related to Caldicellulosiruptor saccharolyticus DSM 8903 (99.0% identity). Physiological and biochemical data also supported that identification of strain F32 as a Caldicellulosiruptor species. The proteins secreted by Caldicellulosiruptor sp. F32 grown on xylan showed a xylanase activity of 7.74 U/mg, which was 2.5 times higher than that of C saccharolyticus DSM 8903. Based on the genomic sequencing data, 2 xylanase genes, JX030400 and JX030401, were identified in Caldicellulosiruptor sp. F32. The xylanase encoded by JX030401 shared 97% identity with Csac_0696 of C saccharolyticus DSM 8903, while that encoded by JX030400 shared 94% identity with Athe_0089 of C bescii DSM 6725, which was not found in the genome of strain DSM 8903. Xylanse encoded by JX030400 had 9-fold higher specific activity than JX030401. Our results indicated that although the 2 strains shared high identity, the xylanase system in Caldicellulosiruptor sp. F32 was more efficient than that in C. saccharolyticus DSM 8903. (C) 2013 Elsevier Inc. All rights reserved.