The crystal structures of thermophilic xylanases from Chaetomium thermophilumandNonomuraea flexuosawere determined at and A˚ resolution, respectively. Both enzymes have the overall fold typical to family 11 xylanases with two highly twistedb-sheets forminga large cleft. The comparison of 12 crystal structures of family 11 xylanases from both mesophilic and thermophilic organisms showed that the structures of different xylanases arevery similar.