The structural gene for a thermostable protease from Bacillus stearothermophilus was cloned in plasmid pTB90. It is expressed in both B. stearothermophilus and Bacillus subtilis. B. stearothermophilus carrying the recombinant plasmid produced about 15-fold more protease (310 U/mg of cell dry weight) than did the wild-type strain of B. stearothermophilus. Some properties of the proteases that have been purified from the transformants of B. stearothermophilus and B. subtilis were examined. No significant difference was observed among the enzyme properties studied here despite the difference in host cells. We found that the protease, neutral in pH characteristics and with a molecular weight of 36,000, retained about 80% of its activity even after treatment of 65 degrees C for 30 min.