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 43kDa and 4.5, respectively. When assayed at 70°C, it was optimally active at pH 9.0–9.5. The optimum temperature for the activity was 75°C at pH 9 and 70°C at pH 10. The enzyme was stable over a broad pH range and showed good thermal stability when incubated at 65°C in pH 9 buffer. The enzyme activity was strongly inhibited by Mn 2+ . Partial inhibition was also observed in the presence of 5mM Cu 2+ , Co 2+ and EDTA. Inhibition by Hg 2+ and dithiothreitol was insignificant. The enzyme was free from cellulase activity and degraded xylan in an endo-fashion.