Streptococcus mutans is capable of resisting acidic pH values found in carious lesions, as low as pH 3.0. Other bacteria, such as Lactococcus lactis, utilize citrate in order to confer protection against the acidic end products of sugar metabolism. Objective: To determine if S. mutans can transport citrate and whether citrate transport plays a role in acid tolerance. Methods: A deletion mutation in the S. mutans citM gene (homologous to a citrate transporter in Bacillus subtilis) was generated by allelic exchange mutagenesis. Assays were performed using cells grown in the presence and absence of citrate to measure the rate of citrate transport, resistance to acidic pH, growth rate, terminal pH of NG8 and mutant (SMCITM) cultures. Results: Citrate transport was demonstrated with the highest rate (203 nmol/mg dry wt./min) measured in cells grown in pH 6.5 medium supplemented with citrate in the presence of Zn 2+, Mg2+, or Ca2+. Only background levels of transport were measured in SMCITM. Using an in vitro acid tolerance assay SMCITM adapted to acid stress as well as NG8 while both SMCITM and NG8 failed to adapt when grown with citrate. SMCITM grew significantly more slowly than the NG8 under all conditions tested (pH 6.0 – 7.5, +/- citrate, glucose or raffinose). The terminal pH of overnight cultures was significantly higher for SMCITM under all conditions tested (pH 6.0 or 7.0, +/- citrate, raffinose or glucose). Conclusion: S. mutans transports citrate at an optimal pH of 6.5. Citrate has a negative effect on the acid tolerance, and growth rate of both SMCITM and NG8. Supported by grant MT-15431 from CIHR of Canada.