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Bacillus sp. MG-33 was isolated from the desert of Rajasthan (India). The organism produced 500 and 200 Ug(-1) of thermostable mannanase (after 96h) in solid state fermentation (SSF) of wheat bran and wheat straw rich-soda pulp at the moisture ratio of 1:1.5 and 1:3 at 30 degrees C, respectively. Two-step partially purified mannanase was optimally active at 65 degrees C and was 100% thermostable at 55 to 60 degrees C for 2h and also retained more than 50% residual activity at 65 degrees C for 2h. A pH of 6.5 was optimum for enzyme activity and 100% stability up to 4h at this pH. Mannanase activity was slightly enhanced by Ca2+, Fe3+, and Mg2+, while 100% activity was retained in the presence of Ba3+, Li+, and NiCl2 at 1.0-10mM. 1M NaCl and urea did not reduce the enzyme activity. The K-m and V-max of mannanase were 0.2mgml(-1) and 60Umg(-1)ml(-1), respectively. Hydrolysis of locust bean was rapid and linear between 5 and 20 min, and similar to 300 mu gml(-1) mannose was obtained after 20 min of catalytic reaction by enzyme at 65 degrees C. TLC was used to confirm the mannose as an end product after hydrolysis of locust bean gum.