Abstract: Heat and acid-stable alpha-amylase enzymes having the following characteristics: (1) capable of retaining at least about 70% of their initial activity when held at 90.degree. C. and at a pH of 6.0 for 10 minutes in the absence of calcium ion; (2) capable of retaining at least about 50% of their initial activity when held at 90.degree. C. at a pH of 6.0 for 60 minutes in the absence of added calcium ion; and/or (3) capable of retaining at least about 50% of their initial activity at a temperature of 80.degree. C. and at a pH of 4.5 in the presence of 5 mM calcium ion for 10 minutes. The preferred alpha-amylases are prepared by culturing a strain of a Bacillus stearothermophilus microorganism in a suitable culture medium. The novel alpha-amylases are useful in hydrolyzing and/or liquefying starch and due to their stability at low pH values they can be used in conjunction with other acid stable amylases such as gluco-amylase in either a soluble or an immobilized form.