A Bacillus subtilis secretion vector was constructed from its α-amylase gene ( amyE ), in which a putative signal peptide was composed of 41 amino acids. A thermostable α-amylase gene ( amyT631 ) from B. stearothermophilus A631 was cloned in a B. subtilis plasmid pUB110. The isolated plasmid was designated as pTUB607. Then amyT631 was introduced into the B. subtilis secretion vector, after amyT631 was partially digested by a exonuclease BAL 31. Chimeric plasmids pTUB613, pTUB616 and pTUB617 were isolated. pTUB613 and pTUB616 encoded Cys residue in the NH₂-terminal regions of the fused amyT631 . B. subtilis strains containing pTUB613, pTUB616 and pTUB617 produced fused extracellular thermostable α-amylases whose molecular weight was estimated to be approximately 63, 000, because the putative signal peptide of amyE was cleaved between 31 and 32 amino acid position from the translation initiator Met. The molecular weight of the parental pTUB607-α-amylase was estimated to be 61, 000. Fused extracellular thermostable α-amylase from pTUB613 and pTUB616 showed an increased thermostability at 90°C, while pTUB607-α-amylase did not. The increased thermostability was suppressed when the pTUB613 and pTUB616-α-amylase were heated in the presence of 100mM mercaptoethanol. The extracellular parental thermostable α-amylase from pTUB607 contained sole Cys residue at position 363. Thus it is suggested that the increase of the thermostability in pTUB613-and pTUB616-α-amylases is closely related to the formation of intramolecular disulfide bonds.