The characterization of a partially purified, cellulase-free, thermostable alkaline xylanase from thermoalkalophilic Bacillus sp. JB-99 was investigated. The xylanase production was the highest when birchwood xylan was added to a medium containing finely powdered rice bran, showing 4,826 IU$ml^-1$ of activity for 15 h of incubation. The partially purified xylanase exhibited an optimum temperature and pH at $70^C{circ}$ and 10, respectively. The enzyme was stable at pH 5-11 at $50^C{circ}$. The xylanase activity was strongly inhibited by $Hg^2+$, while dithiothreitol, cysteine, and ${eta}$-mercaptoethanol enhanced the activity.