Bacillus subtilis RM615, isolated from the house roach, produced extracellular proteases on a skim milk plate. The major protease (AprA) was purified by 80% acetone precipitation and DE and CM-cellulose chromatography. The purified enzyme had a molecular weight of 28,000 daltons on SDS-PAGE. The enzyme was inhibited by phenylmethane-sulphonyl fluoride (PMSF) and soybean trypsin inhibitor (STI), but not by ethylene diamine tetra-acetic acid (EDTA), o-phenanthroline, elastatinal, histidine, or leupeptin. These results indicated that it is a serine protease. This protease preferentially cleaved Leu or Arg as the P1 site. The proteolytic activity of AprA was reduced to 75% of maximum activity by treatment with 5 M urea for 1 h at $25^{circ}C$, and to 82% by treatment with 1% sodium dodecyl sulfate (SOS). The protease showed high enzyme activity in a wide pH range from 6.0 to 12.0, and was stable in alkali, retaining 85% of its initial activity at pH 12.0 after 24 h incubation period at $25^{circ}C$. Enzyme activity reached a maximum at approximately $60^{circ}C$. When the enzyme was incubated for 30 min at pH 11.0 (50 mM CAPS buffer, 10 mM $CaCl_2$) at $70^{circ}C$, the residual activity of the enzyme was 71%.