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Ten thermophilic Gram-positive bacteria were isolated from various soils of Saudi Arabia. The strains are spore-forming rods belonging to the species Bacillus stearothermophilus. The cells are motile, strictly aerobic, catalase and oxidase positive. The sporangia appear to be swollen and their position varies from terminal in some to sub-terminal in others. The thermal stability of some enzymes of these bacteria was investigated; extracellular alpha-amylase appears to be very sensitive to pH and temperature. The ultrastructure of these bacteria shows specific changes in the cell wall when grown at the maximum, minimum and optimum growth temperatures, respectively.