Six strains of Prevotella bivia and 4 of Gardnerella vaginalis were examined for nutrient substrate utilization as part of ongoing studies on the pathogenesis of bacterial vaginosis. Addition of single amino acids to vaginal defined medium (VDM) was stimulatory to the growth of P. bivia but not to G. vaginalis. However, peptides significantly promoted the growth of both organisms. Growth of P. bivia in VDM and VDM supplemented with either amino acids or peptone was accompanied by net ammonia production, while growth of G. vaginalis under the same conditions resulted in net ammonia utilization. Ammonia-enriched supernatants from the growth of P. bivia in peptone-supplemented VDM were stimulatory to G. vaginalis growth. However, ammonia-reduced supernatants from G. vaginalis growth in peptone-supplemented VDM had a neutral effect on P. bivia growth. A commensal relationship between P. bivia to G. vaginalis is proposed, with ammonia flow as a mechanism to support this hypothesis.