Most likely, young children acquire their oral microflora by frequent transfer of bacteria between family members. The possible transmission of obligately anaerobic Prevotella melaninogenica recovered from 11 mother-child pairs was examined by ribotyping. One to 18 isolates (mean 13) per child from different oral sampling sites and 4 to 17 (mean 10) isolates per mother from stimulated salivary samples, collected on 2 occasions, were analyzed. On sampling, the mean ages of the children were 4 months and 32 months, respectively. Restriction endonucleases KpnI and ClaI were chosen for the digestion of chromosomal DNA. DNA fragments were electrophoretically separated, blotted onto a nylon membrane and hybridized with rRNA operon of Escherichia coli. DNA-DNA hybrids were detected immunologically. Extensive genetic heterogeneity, 101 distinct ribotypes, was observed among 248 P. melaninogenica isolates studied. Both mothers and children harbored several (up to 7) ribotypes which, apart from 3 ribotypes, were distinguishable in unrelated subjects. Several P. melaninogenica ribotypes were detected on both sampling occasions over 2 years apart. Identical ribotypes were found in 6 of the 11 mother-child pairs, 1 to 2 similar ribotypes per pair. This suggests the transmission of P. melaninogenica between the mother and her child, probably via maternal saliva. However, the unique ribotypes found in these children also indicate that other sources besides the mother influence the oral colonization of young children.