Lactic acid is the major end-product of glycolysis by Streptococcus mutans under conditions of sugar excess or low environmental pH. However, the mechanism of lactic acid excretion by S. mutans is unknown. To characterize lactic acid efflux in S. mutans the transmembrane movement of radiolabelled lactate was monitored in de-energized cells. Lactate was found to equilibrate across the membrane in accordance with artificially imposed transmembrane pH gradient (Δψ). The imposition of a transmembrane electrical potential (Δψ) upon de-energized cells did not cause an accumulation of lactate within the cell. The efflux of lactate from lactate-loaded, deenergized cells created a ΔpH, but did not create a Δψ, indicating that lactate crosses the cell membrane in an electroneutral process, as lactic acid. ΔpH and Δψ were determined by the transmembrane equilibration of [¹⁴C]benzoic acid and [¹⁴C]tetraphenylphosphonium ion (TPP), respectively. The presence of a membrane carrier for lactic acid in S. mutans was suggested by counterflow. Enzymic determination of the intra- and extracellular lactate concentrations of S. mutans cells glycolysing at pH_o 6.8 and 5.5 showed that lactate distributed across the cell membrane in accordance with the equation ΔpH = log[lact]_i/[lact]_o. The addition of high extracellular concentrations of lactate to glycolysing S. mutans at acidic pH resulted in a fall in ΔpH and a subsequent decrease in glycolysis. The fall in ΔpH was attributed to the F₁F₀ ATPase being unable to raise the pH_i back to its initial level due to the build up of lactate anion within the cell creating a large Δψ. The increase in Δψ resulted in the overall proton motive force remaining constant at about −110 mV. The results demonstrate that lactate is transported across the cell membrane of S. mutans as lactic acid in an electroneutral process that is independent of metabolic energy and as such has important bioenergetic implications for the cell.