The oral establishment of various species of streptococci and lactobacilli has been studied on 25 infants during a period of 5 years. When the children were 3, 4 and 5 years old, their caries and gingival status were recorded. Streptococcus salivarius becomes established within a day of birth, and Streptococcus sanguis after the eruption of the teeth during the first year. Streptococcus mutans was recovered from the infants significantly later and had only become established in half the number of the infants when they had reached 5 years. Lactobacilli were only recovered in low numbers and below 2 years of age they seemed to be mostly transients in the mouth of the infants. The lactobacillus flora which was then isolated, was dominated by Lactobacillus casei and the composition of the flora was significantly different from that which has previously been described for adults.