The maintenance of a low pH in the vagina through the microbial production of lactic acid is known to be an important defense against infectious disease in reproductive age women. Previous studies have shown that this is largely accomplished through the metabolism of lactic acid bacteria, primarily species of Lactobacillus. Despite the importance of this defense mechanism to women's health, differences in the species composition of vaginal bacterial communities among women have not been well defined, nor is it known if and how these differences might be linked to differences in the risk of infection. In this study, we defined and compared the species composition of vaginal bacterial communities in 144 Caucasian and black women in North America. This was carried out based on the profiles of terminal restriction fragments of 16S rRNA genes, and phylogenetic analysis of 16S rRNA gene sequences of the numerically dominant microbial populations. Among all the women sampled, there were eight major kinds of vaginal communities ('supergroups') that occurred in the general populace at a frequency of at least 0.05 (P=0.99). From the distribution of these supergroups among women, it was possible to draw several conclusions. First, there were striking, statistically significant differences (P=0.0) in the rank abundance of community types among women in these racial groups. Second, the incidence of vaginal communities in which lactobacilli were not dominant was higher in black women (33%) as compared to Caucasian women (7%). Communities not dominated by lactobacilli had Atopobium and a diverse array of phylotypes from the order Clostridiales. Third, communities dominated by roughly equal numbers of more than one species of Lactobacillus were rare in black women, but common in Caucasian women. We postulate that because of these differences in composition, not all vaginal communities are equally resilient, and that differences in the vaginal microbiota of Caucasian and black women may at least partly account for known disparities in the susceptibility of women in these racial groups to bacterial vaginosis and sexually transmitted diseases.