This study presents the effects of citric acid and extracellular pH (pH(e)) on the intracellular pH (pH(i)) of wild-type and citrate negative variants (cit(-)) Leuconostoc mesenteroides subsp. mesenteroides (Ln. mesenteroides M) and Lactococcus lactis subsp. lactis bv. diacetylactis (L. lactis LD). A recent method using a pH-sensitive fluorescent indicator carboxyfluorescein succinimidyl ester (cFSE) was adapted to measure the pH(i) of these two lactic acid bacteria in resting cells. Energized cells with 10 mM lactose of Ln. mesenteroides M and L. lactis LD modified their pH gradient (Delta pH) in the same manner; when the pH(e) was decreased from 7 to 4, the pH(i) decreased from 7 to about 5. The adjunction of 10 mM citric acid had no effect on the pH(i) of wild-type and cit(-) variant of L. lactis LD, nor on the pH(i) of Ln. mesenteroides cit(-) variant. Nevertheless, in Ln. mesenteroides M wild-type, citric acid utilization increased the pH(i), which was maintained at about 6.5-7.0 when the pH(e) was decreased from 7 to 4. It could be concluded that citric acid allows the maintenance of pH homeostasis in Leuconostoc mesenteroides.