Abstract: Objective　To find out horizontal transmission of oralStreptococcus mutans(S.mutans) in nursery children through analyzing the similarity ofS.mutansgenotypes.Methods　The study group included 24 nursery children between 3 and 4 years of age. Dental plaque sampleswere collectedwith sterile toothpick and cultured onMSB plates for48 h. IndividualStreptococcus mu- tansgroup colonies representative of the colonial morphologies were subcultured on TPY plates. These strains were identified to species level biochemically. AP-PCR fingerprinting was preformed after identification.S.mutansisolates from different children with very similar fingerprinting profiles were examined by chromosomal DNA fingerprinting analysis.Results　Streptococcus mu- tansgroup were isolated in oral cavities of 66.7% children, 58.3% in caries-free and 75.0% in caries children. A total of 46 S.mutansisolates from 24 subjects were analyzed by AP-PCR, and 29 different amplitypes were identifyied, 45.8% carried two genotypes. There were 2 genotypes ofS.mutansisolated repeatedly among 12 nursery children.Conclusion　The presence of matching genotypes ofS.mutansamong nursery children suggests horizontal transmission.

Key words: Streptococcus mutans, genotype, arbitrarily primed polymerase chain reaction, DNA fingerprinting