The repressor gene and its allele--a temperature sensitive mutant have been cloned from the defective prophage PBSX of Bacillus subtilis by means of PCR technique. The characterization of these sequences suggest that wild-type and mutant gene have the same open reading frame, which probably encode a repressor protein of 113 amino acids, and the putative promoter region and ribosome binding sites within the sequences have also been identified. The results from complementation experiments demonstrate that repressor encoded by wild-type gene is capable of blocking the temperature-shift to induce PBSX prophage in Bacillus subtilis.