The effect of pH, mental ions, and denaturing reagents on the thermal stability of thermophilic alpha-amylase [EC 3.2.1.1] were examined. The enzyme was most stable at around pH 9.2, which is coincident with the isoelectric point of the enzyme. The stability of the enzyme was increased by the addition of calcium, strontium, and sodium ions. The addition of calcium ions markedly stabilized the enzyme. The protective effects of calcium and sodium ions were additive. At room temperature, no detectable destruction of the helical structure of the enzyme was observed after incubation for 1 hr in the presence of 1% sodium dodecylsulfate, 8 M urea or 6 M guanidine-HC1. The addition of 8 M urea or 6 M guanidine-HC1 lowered the thermal denaturation temperature of the enzyme. The enzyme contained one atom of tightly bound intrinsic calcium per molecule which could not be removed by electrodialysis unless the enzyme was denatured. The rate constants of inactivation and denaturation reactions in the absence and presence of calcium ions were measured and thermodynamic parameters were determined. The presence of calcium ions caused a remarkable decrease in the activation entropy.