Our objective was to identify bacterial species present in culture-negative but 16S rDNA-positive amniotic fluid samples from women in preterm labor. Amniotic fluid from 69 women in preterm labor was cultured and examined for the proinflammatory cytokine interleukin-6 (IL-6). Polymerase chain reaction technology was used to detect highly conserved bacterial ribosomal DNA sequences (16S rDNAs). As previously reported, 16S rDNAs were identified in 15 (94%) of 16 culture-positive amniotic fluid samples, in 5 (36%) of 14 culture-negative samples with elevated IL-6, and in 1 (3%) of 39 culture-negative samples with low IL-6 levels. Direct sequencing was performed of 16S rDNAs from the 5 culture-negative amniotic fluid specimens with elevated IL-6, followed by database searches and phylogenetic analyses. The bacterial sequences identified included: two Leptotrichia sanguinegens, one human oral bacterium A33, one Fusobacterium nucleatum, and one Ureaplasma urealyticum. Identification and sequencing of 16S rDNAs in amniotic fluid is a promising technique to identify bacterial species associated with elevated IL-6 levels in culture-negative amniotic fluid that may contribute to the etiology of premature labor.