Summary A bank of pTV32 (Tn 917 1acZ) — generated Bacillus sbutilis mutants were examined on milk agar for the ability to produce proteases at 48°C. A single mutant, BUL786, was isolated, which could hydrolyze casein after overnight incubation at 48°C. This mutant secreted protease 10 fold more at 48°C when compared to 37°C, and part of the activity appears to be 48°C-specific. At high temperatures, other strains of B. subtilis, including hyperprotease secretors, were unable to secrete protease to any significant degree. The BUL786 strain is missing the 97K major heat shock protein. Since a number of other proteins also appear to be secreted at 48°C, this mutant may be a hypersecretor of exported proteins at temperatures > 45°C.