A thermostable, alkaline active xylanase was purified to homogeneity from the culture supernatant of an alkaliphilic Bacillus halodurans S7, which was isolated from a soda lake in the Ethiopian Rift Valley. The molecular weight and the pI of this enzyme were estimated to be around 43 kDa and 4.5, respectively. When assayed at 70 degrees C, it was optimally active at pH 9.0-9.5. The optimum temperature for the activity was 75 degrees C at pH 9 and 70 degrees C at pH 10. The enzyme was stable over a broad pH range and showed good thermal stability when incubated at 65 degrees C in pH 9 buffer. The enzyme activity was strongly inhibited by Mn2+. Partial inhibition was also observed in the presence of 5 mM Cu2+ Co2+ and EDTA. Inhibition by Hg2+ and dithiothreitol was insignificant. The enzyme was free from cellulase activity and degraded xylan in an endo-fashion. (c) 2006 Elsevier Inc. All rights reserved.