The relative viability of X-bearing and Y-bearing spermatozoa is influenced by factors in the vagina such as pH. The vaginal environment, in turn, is influenced by its flora. This study examined the relationship of the vaginal flora to the sex of conceptuses. It was found that women who carried Trichomonas vaginalis or Bacteroides sp or who had nonspecific vaginitis at first prenatal visit were significantly more likely to deliver females than women who carried none of these organisms (54 versus 37%, P less than .02).