The present invention describes a method for producing butanol by fermentation of carbohydrates using mixed populations of acidogenic-phase cells and solventogenic-phase cells of Clostridium in a solitary vessel. The present system as described does not require intermittent adjustment of pH or venting of headspace gases. The method provides a process for removal of the butanol product which does not irreversibly harm the cells and conditions are described where such cells may resume butanol synthesis in the same solitary vessel. The invention also describes compositions and biologically pure cultures which comprise the Clostridium cells as disclosed.