Abstract: Butanol and acetone are major end products of a bacterial fermentation process carried out by anaerobic saccharolytic clostridia such as Clostridium acetobutylicum. However, these organisms degenerate upon repeated subculture of growing cells, thereby losing their capacity to produce solvents and to develop into spores. A novel class of degeneration-resistant mutants of C. acetobutylicum, generated by transposon insertion mutagenesis, has been produced employing a novel selection procedure. Upon subculture the mutant cells survive significantly longer than the parental cells without altered solventogenic characteristics. The mutant organism, the technique for its isolation and its use are provided.