The purposes of this study were to determine the age at the initial acquisition of mutans streptococci (MS) and to determine the transmission of MS among children at day nursery by describing the occurrence of genotypes prepared by chromosomal DNA fingerprinting of the bacteria using restriction endonuclease EcoRI and HaeIII. The samples were 39 children (23 boys and 16 girls) aged 0-5 years old, 14 pairs of parents and 6 nursery caretakers of a day nursery in Hiroshima city, Japan. The children had no dental caries throughout the experiment. Plaque samples of the children were taken using toothbrushes at 1 month intervals for 30 months. The initial acquisition of MS occurred between the ages of 8 months and 52 months with a mean age of 24.2 months. The cumulative probability of initial acquisition of MS increased rapidly at the age of 12 to 25 months after 10 to 20 primary teeth had erupted. Transmission of MS was found between child and mother (33.3%), child and father (8.3%) and child and others including amongst the children (58.4%), but no evidence of MS transmission from nursery caretakers was found. Two children acquired MS from intra- and extra-familial transmission. This study suggests that the child's environment also plays a role in the initial acquisition and transmission of MS, in addition to the oral condition of the children.