Objective Genital tract infection accounts for approximately 25-40% of all preterm births. We sought to assess the relationship between preterm birth and selected vaginal bacterial taxa associated with preterm birth either directly or through their association with bacterial vaginosis (BV). Study Design Vaginal fluid for Gram stain was collected between 17 and 22 weeks' gestation as part of a randomized trial of ultrasound-indicated cerclage for preterm birth prevention in women at high risk for recurrent spontaneous preterm birth. Bacterial deoxyribonucleic acid was extracted from the Gram stain slides and analyzed using quantitative polymerase chain reaction. Results Among the 499 participants, Mycoplasma was positively correlated with increased risk of preterm (risk ratio [RR], 1.83; 95% confidence interval [CI], 1.52-2.22) as was Mobiluncus (RR, 1.36; 95% CI, 1.07-1.73) and Atopobium (RR, 1.44; 95% CI, 1.1-1.87). However, there were strong interactions between the race/ethnic group and the presence of these and other individual taxa on risk of preterm birth. By contrast, bacterial vaginosis-associated bacteria (BVAB)-3 was consistently associated with a reduction in the risk of preterm birth for all racial/ethnic groups (0.55; 95% CI, 0.39-0.78). Conclusion BV is characterized by a reduction of Lactobacillus, and lactic acid-producing bacteria and the presence of Mobiluncus; we found these factors and the presence of Mycoplasma to be associated with an increased risk of preterm birth. By contrast, the presence of a recently identified organism sufficient to cause BV, BVAB3, decreased the risk of preterm birth. These findings give insight into why treating BV has mixed impact on risk of preterm birth.