Alkaliphilic and thermophilic Bacillus sp. DM-15 was isolated from ciftehan thermal spring, Turkey, produced thermostable xylanase at 55 degrees C at pH 9. The molecular weight of the enzyme was estimated to be 95.6 kDa by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The enzyme showed broad working pH range of 4.5-10.0 with art optimum pH of 6.5. The temperature optimum of the enzyme was found to be 60-70 degrees C. Xylanase production by thermophilic Bacillus sp. strain cultivated in liquid media reached a maximum at 36h, with levels of 192.8 mu mol mg(-1) protein/min. The enzyme was stable for 15 min at 60 degrees C while 13, 86, 95 and 96% of the original activities were lost at 70, 80, 90 and 100 C, respectively. Enzyme activity was increased in the presence of 5 mkt CaCl2 (137%), CoCl2 (133%), CuSO4 (112%), MgCl2 (106%) and KCl (105%) and inhibited in the presence of 5 mMEDTA, HgCl2, FeSO4, SDS (1%), MnCl2, Triton X-100 (1%) up to 93, 89, 73, 41,29 and 8%, respectively. The DM-15 thermostable xylanase may be suitable for several industrial applications, such as food, agricultural and biofuel.