An aroma-imparting mesophilic lactic starter (Lactococcus lactis ssp. lactis biovar. diacetylactis) was studied in batch culture in medium with 50 g.l(-1) lactose and 2 g.l(-1) citrate. The effect of pH on the physiology of growth and the production of flavour compounds was investigated with a mathematical model. The specific rates of growth and of lactose fermentation obeyed a law of non-competitive inhibition by lactic acid produced, inhibition increasing as the pH of the medium decreased. The pH thus acted indirectly by increasing the proportion of non-dissociated lactic acid, identified as the inhibiting form of lactic acid. The generalized model, taking into account the effect of pH, was tested using fermentations at pH controlled at different values (4.5-6.5), as well as with a fermentation conducted at non-regulated pH. These simulations supported the working hypotheses. The effect of pH on the fermentation of citric acid resulted in an increase in the maximal specific rate of citrate utilization, in the bioconversion yield, and in the constant of diacetyl and acetoin reduction at acid pH. The production of flavour compounds is a complex phenomenon resulting from the interaction of pH, citric acid concentration, and the physiological state of the cells. These results are discussed with respect to the use of this strain in the preparation of manufactured dairy products.