The qualitative and quantitative a-amylase production capacities of six thermophilic bacilli were screened. Geobacillus sp. D413 was selected for enzyme optimization, as it displayed higher alpha-amylase activity. The maximum enzyme activities of D413 and G. stearothermophilus ATCC 12980(T) were observed at the time of 72 h. While the optimal pH of medium for bacterial growth and enzyme production of D413 (pH 7.0) differed from ATCC 12980(T) (pH 8.0), the optimal temperature for enzyme production was 55 degrees C for both. The effects of various carbon and nitrogen sources were determined by changing their concentrations. The highest bacterial growth and enzyme production were sustained by the starch and maltose containing medium. Both bacterial growth and enzyme production were inhibited by NH4Cl. D413 and ATCC 12980T amylases showed optimal activity at 65 degrees C, pH 9.0 and at 65 degrees C, pH 7.5, respectively. They remained active over temperature and pH ranges of 45-75 degrees C and 4.0-10.5. Their activities retained 65% and 54% when incubated at 75 degrees C for 10 min and 98-86.5% and 95-84.5% at pH 4.0-10.5 for 15 h at 37 degrees C. In conclusion, the alpha-amylase production conditions of D413 have been optimized which can be useful in biotechnological processes such as hydrolysis of starch to glucose.