Using a-amylase as an example, extremely thermophilic Bacilli isolated from heat-treated sewage sludge are shown to be a source for enzymes stable and active at high temperatures. The isolates which are classified as subspecies of Bacillus stearothermophilus differ from each other in protein composition indicating the heterogeneiety of that subspecies. Media are evaluated for good growth and high enzyme productivity. Best media are those composed of three or four different complex components like combinations of peptone, soy grist, and malt extract, a-amylase production on simple carbon sources is negligible. From the cultivation supernatants crude a-amylase extracts are prepared and their behaviour at high temperatures is described. The optimal temperature of all tested enzymes is 80 ยฐ C. They are stable at suboptimal temperatures for over 20 h and at 95 ยฐ C 50% of their activity is lost within 2 h. The activity at 95ยฐC is however preserved for over 3 h in presence of starch. The products of the starch digestion are maltotriose, maltose, and some glucose. The amylases can therefore compete in activity and stability with commercially available a-amylases from Bacillus licheniformis.