Children are more resistant to gingivitis than are adults. To determine possible differences in their periodontal floras, an experimental gingivitis study, identical in design to one reported earlier with young adults, was conducted with four 4- to 6-year-old children. The incidence of sites that developed gingival index scores of 2 in children was less than one-third of the incidence observed in adults. The composition of the flora of each child was statistically significantly different from that of any other child or adult. The floras of the children as a group were statistically significantly different from those of the adults. Children had 3-fold greater proportions of Leptotrichia species, 2.5-fold greater proportions of Capnocytophaga species, 2.3-fold greater proportions of Selenomonas species, 2-fold greater proportions of bacterial species that require formate and fumarate, and 1.5-fold greater proportions of Bacteroides species. Adults had greater proportions of Fusobacterium, Eubacterium, and Lactobacillus species. Fusobacterium nucleatum, Actinomyces WVa 963, Selenomonas D04, and Treponema socranskii were predominant species that correlated with increasing gingival index scores in both children and adults.