In order to understand the acid tolerance mechanism in Clostridium acetobutylicum against the undissociated weak acids, a two stage continuous culture of C. acetobutylicum KCTC 1037 was made to supply acidogenic and solventogenic phase culture for a study on the protonmotive force and fermentation profile in the presence of butyrate, the acidic fermentation product. Undissociated butyric acid was permeable to the membrane of acidogenic cell, while the influx of the acid was minimal into the solventogenic cell. Acidogenic cell had higher pH gradient and protonmotive force than solventogenic cell. The protonmotive forces decreased linearly with the increase in butyrate from 25 to 75 mM. Acidogenic culture maintained the internal pH of 5.6-6.2 in the presence of 75 mM butyrate, and the internal pH of the solventogenic cultures was 5.3-5.7 in the similar conditions. Though the acidogenic culture was found to maintain the protonmotive force in the presence of 75 mM butyrate in the short term experiment, a long term incubation showed that the acidogenic culture was unable to ferment sugar in the similar conditons. On the other hand the solventogenic culture maintained active protonmotive force and fermented sugar in the presence of 75 mM butyrate. The protonmotive force of acidogenic culture was dissipated by 75 mM butyrate in the presence of ATPase inhibitor, dicyclohexylcarbodiimide, whilst the protonmotive force of solventogenic culture was maintained a similar condition. From these results, it is concluded that the acidification of cytoplasm by butyric acid is prevented by proton pumping action of the membrane bound ATPase which maintains the protonmotive force for a short-term period in the acidogenic cultures, and that solventogenic culture possess the ability to reduce the influx of organic acid in long term fermentation. It is suggested that C. acetobutylicum maintains the protonmotive force by the investment of ATP during the fast-fermenting acidogenesis; on the other hand, the solventogenic culture can reduce the influx of the acidic fermentation products to maintain the protonmotive force.