The vaginal flora of 118 Japanese and 208 Thai pregnant women were investigated for the presence of bacterial vaginosis (BV), BV-associated organisms, and BV-associated enzyme. A similar prevalence of BV was found among the Japanese (13.6%) and Thai women (15.9%). The microbial flora of women with BV were complex; the mean number of isolates recovered in the BV group was approximately 2 times more than that in a group of healthy women. Prevotella species, Porphyromonas species, Peptostreptococcus species, Mobiluncus species, Gardnerella vaginalis, and H2O2-nonproducing lactobacilli were significantly associated with BV. These organisms were less associated with H2O2-producing lactobacilli, which were predominant in women with normal flora, suggesting that H2O2-producing lactobacilli have antibacterial activity against BV-associated organisms. The vaginal sialidase assay by means of a filter-paper spot test was not proved to be a useful screening aid for diagnosis of BV because of the low sensitivity (69.4%) of this test.