The susceptibility of Mycobacterium tuberculosis and Mycobacterium smegmatis to a range of weak acids and acid pH was investigated. M. tuberculosis was found to be more susceptible to acid pH and weak acids than M. smegmatis. The weak acids were more active against M. tuberculosis at acid pH than at neutral pH. M. tuberculosis was found to be less able to maintain its internal pH and membrane potential at acid pH than M. smegmatis. The antituberculous activity of weak acids correlated with their ability to disrupt the membrane potential but not the internal pH. The significance of these findings is discussed in relation to M. tuberculosis physiology and development of new antituberculous agents.