Quantitative bacteriology was performed on vaginal secretions from healthy adult women. The analysis included a single sample from 17 college students and 35 samples from five volunteers collected at intervals of three to five days throughout the menstrual cycle. Mean concentrations in all 52 specimens were 10(8.1) aerobic bacteria/g and 10(9.1) anaerobic bacteria/g. The rank of predominant organisms, according to rates of recovery in concentrations of greater than 10(5) colony-forming units/g, was anaerobic and facultative Lacrobacillus species, Peptococcus species, Bacteroides species, Staphylococcus epidermidis, Corynebacterium species, Peotostreptococcus species, and Eubacterium species. Sequential samples collected throughout the menstrual cycle showed relatively consistent mean levels of anaerobes and a significant decrease in concentrations of aerobes in premenstrual specimens compared with those in the specimens collected in the week following onset of menses. Analysis of sequential specimens from each of the five individuals showed considerable variation in species recovered. These data indicate that the vaginal flora in healthy adult women is a dynamic ecosystem in which anaerobes are usually the numerically dominant bacteria.