Growth and acid production coupling for Lactobacillus helveticus was found to be mainly controlled by cultivation pH. The kinetics of the two processes were uncoupled at acidic pH. This conclusion has been established by means of a simple plot, under conditions where the nitrogen source was not limiting. During the deceleration phase, the acid production rate remained close to its maximal value at a given cultivation pH, while growth rate decreased notably. Both growth and acid production were inhibited by undissociated lactic acid accumulated in the medium, but the growth rate was shown to be more affected than acid production. For the strain of L helveticus used throughout this work, irrespective of cultivation pH, concentrations of free lactic acid of 8.5 and 13.4 g dm(-3) resulted in a full inhibition of growth (leading to a stationary phase) and acid production respectively. These findings will be useful to optimize the mean volumetric acid productivity of batch fermentations, both at constant and programmed pH values.