A mutant strain which hyperproduced thermostable α-amylase was obtained by chemical mutagenesis of Bacillus licheniformis. The mutant strain, SK-5, produced the enzyme about 50 times higher than the original strain. The mutant was longer and slimmer in shape, slower in growth compared to the original strain. Nucleotide sequence analysis of the SK-5 α-amylase gene revealed no changes in the the structural gene. The changes found in the promoter region might be responsible for the hyperproduction of the enzyme by the mutant. No structural changes in the enzyme structure could be observed when the, secreted enzymes at various culture times were analyzed by Western blot.