Bacillus sp. MK716, a thermophilic and amylolytic soil isolate, produced a thermostable alpha-amylase active even at 100 degrees C. The enzyme yield was increased 40 times through mutagenesis with ethylmethane sulfonate. The mutated gene was cloned in Escherichia coli-pBR322 system, and the 4.8 kb cloned fragment was mapped with restriction enzymes. Subcloning in Bacillus subtilis of a 2.0 kb portion of the cloned fragment resulted in 107 times increased enzyme yield. The recombinant plasmid was completely stable in B. subtilis.