The solventogenic sol operon consisting of bld, ctfA, ctfB, and adc was cloned from Clostridium saccharoperbutylacetonicum strain N1-4. These genes share as high as 95-98% similarity with the corresponding sol genes of Clostridium beijerinckii NCIMB 8052. The N1-4 sol gene cluster was transcribed in a polycistronic manner under the control of two promoters, and its transcription was highly induced during solventogenesis. Strain DGN3-4, the degenerated strain derived from N1-4, maintained the sol genes, but transcription of the DGN3-4 sol operon was hardly induced during solventogenesis. A substance extracted from the culture supernatants of wild-type N1-4 allowed us to induce transcription of the sol operon in DGN3-4. These results suggest that the degeneration is caused by the incompetence of the induction mechanism of the sol operon, and that transcription might be under the control of a quorum-sensing mechanism.