When Streptococcus mutans GS5 was grown in equimolar (5 mM) amounts of glucose and lactose, a classical diauxic growth curve was obtained. Glucose was taken up during the first growth phase, followed by a 60-min lag, and then lactose was transported. Synthesis of lactose phosphotransferase system (PTS) enzymes was repressed until the complete exhaustion of glucose, indicative of an inducer exclusion mechanism of repression. The enzyme phospho-beta-galactosidase, however, was found in small amounts even in the presence of glucose. Repression was not observed when GS5 was grown in equimolar amounts of fructose and lactose. Although fructose was taken up preferentially, synthesis of the lactose PTS occurred from the onset of growth in these sugars. It is proposed that a component of the glucose PTS may be a regulatory factor in lactose transport. Glucose PTS- mutants did not display diauxic growth in glucoselactose mixtures and, in fact, transported the disaccharide preferentially.