Gnotobiotic rats infected with Streptococcus mutans 6715, mutant C211 at 45 days of age and provided a purified diet containing 5% sucrose developed carious lesions on buccal, sulcal, and proximal molar surfaces within 15 days (60 days of age). The level of caries increased significantly (P ≤0.01) within the next 15 days (by day 75), and extensive decay was observed on all three molar surfaces of 90 day old infected rats (45 days after challenge). Mutant C211 was previously shown to exhibit increased glucosyltransferase activity and greater adherence and virulence than S. mutans 6715 wild type (wt). Gnotobiotic rats (90 days of age) infected with either S. mutans AHT or S. mutans 6715 (wt) at 45 days of age developed significantly (P ≤0.01) fewer caries on all molar surfaces than rats of the same age that were infected with S. mutans 6715, mutant C211. The level of plaque increased 2 fold and the number of viable S. mutans in plaque increased 10 fold between days 60 and 90 in rats infected with S. mutans 6715, mutant C211. Ninety day old rats infected with either S mutans AHT or S. mutans 6715 (wt) had similar levels of plaque and numbers of S. mutans in plaque; however, these values were two to fourfold lower than those observed in rats of the same age that were infected with S. mutans 6715, mutant C211.