There were three types of industrial acetone-butanol fermentation of blackstrap molasses; one of them, called B type, was characterized by the extremely prolonged acidity peak, and sluggishness experiments were carried out to find some symbiotic organisms among various aerobic bacteria and yeasts for several strains of Clostridium acetobutylicum. Torula utilis showed an outstanding effectiveness for a rapid completion of the fermentation, and the yields of solvents was much increased. Culture filtrate of T. utilis contained a soluble and invertase activity. A close relation was found between high yields of solvents and the degree of inversion of molasses medium. Thus, the effective factor against sluggishness was ascribed to the invertase activity of the yeast. Some inhibiting factors to invertase of C. acetobutylicum were presumed to be present in molasses as the principal cause of the sluggishness.