Objective: To investigate the distribution of Prevotella intermedia in subjects with relatively normal periodontal status and to explore the possible transmission of this species among family members. Methods: Dental plaque samples were taken from 61 children and their 120 parents of 60 families. Culture technique was used to determine the presence of black-pigmenting organisms in subgingival and supragingival plaque samples. Identification of Prevotella intermedia was first performed by traditional biochemical tests and then by oligonucleotide probe hybridization. Arbitrarily primed polymerase chain reaction (AP-PCR) was used for clonal analysis of Prevotella intermedia isolated with primer OPA-03. Results: Prevotella intermedia was identified through biochemical tests in 212 bacterial isolates from 95 subjects. Of these 212 isolates from 44 adults and 35 children, 163 were further identified as Prevotella intermedia by DNA hybridization. AP-PCR was performed for the 73 isolated strains of Prevotella intermedia from 59 individuals of 20 families. A total of 39 distinct amplitypes was detected. In 18 married couples, 8 pairs harbored Prevotella intermedia with indistinguishable AP-PCR patterns. There were 2 families with same amplitypes among children and their parents and 7 families with same amplitypes among children and one of their parents. Four isolates from 4 unrelated individuals shared similar AP-PCR types. Conclusion: Results showed that considerable genetic heterogeneity of Prevotella intermedia exists within unrelated individuals, and indistinguishable amplitypes were found between parents and children as well as between spouses. This study indicates that intrafamilial transmission of Prevotella intermedia may be a pathway for colonization by this species.