Vaginal swabs for microbiological culture were taken from 174 pregnant women whose vaginal flora had been evaluated by Gram's stain; 50 had grade III flora (bacterial vaginosis, BV), 50 grade II (intermediate), 41 had vaginal flora graded as abnormal which then reverted to grade I (revertants) and 33 had normal flora (controls). The aim was to determine whether bacterial species isolated from women with grade II flora differed from those with grade III flora. Isolation of Lactobacillus spp. decreased from grade I to grade III and that of other aerobic and anaerobic bacterial species increased. There was little difference in the species isolated from women with grade II and grade III flora, but there was a distinct order in which organisms in different species increased in numbers. The vaginal flora of revertants was intermediate between that of healthy controls and those with grade II flora. Coagulase-negative Staphylococcus spp. were isolated from a greater number of revertants than grade I controls but the incidence did not increase in grade II or grade III. Bifidobacterium spp. were isolated from a greater number of revertants than grade I controls and increased further in grade II and grade III. However, Gardnerella vaginalis and Mycoplasma hominis were isolated from a much larger number of women with grade III flora than the other groups. The conclusion is that grade II is a transitional phase between grade I and grade III and that some organisms such as G. vaginalis and M. hominis only reach large numbers in the late stage. The sequence of appearance of the various bacterial species may be a result of the pathophysiological alteration of the vaginal ecosystem associated with BV.