When Streptococcus bovis JB1 and Lactococcus lactis ML3 were grown with an excess of glucose, lactic acid accumulation caused a decrease in extracellular pH; S. bovis grew at extracellular pH values as low as 4.9, but L. lactis was unable to grow below pH 5.3. Because both bacteria maintained a low Delta pH across the cell membrane, it appeared that intracellular pH was controlling their pH sensitivities. S. bovis glycolyzed glucose and maintained high concentrations of ATP at intracellular pH values as low as 5.4. L. lactis could not glycolyze glucose when the intracellular pH was less than 5.6, and ATP declined. L. lactis cells that were washed and incubated in buffers with an excess glucose had higher Delta pH values than growing cells. Lactic acid addition, however, prevented the interconversion of membrane potential (Delta Psi) and chemical gradient of protons (Z Delta pH).