It is well known that the number of Streptococcus mutans is closely related with dental caries activity. In this study, the capability of glucan production and the glucosyltransferase (gtf) gene expression were examined in the saliva and plaque samples from caries-active and caries-free children. Bacterial cells from each sample were estimated by viable cell count on selective media. The capability of glucan production was determined by measuring the amount of glucan synthesized on orthodontic wires in a medium inoculated with S. mutans isolates from both groups. The mRNA expression for the gtfB and gtfD genes was examined by Fluorescent In Situ Hybridization (FISH). In accordance with previous reports, the numbers of mutans streptococci (MS) and Lactobacilli were higher in caries-active saliva and plaque than in caries-free samples. S. mutans isolated from caries-active samples produced a large amount of glucan, whereas S. mutans from caries-free samples produced little glucan on the orthodontic wires. Finally, the expression of the gtfB and gtfD genes in caries-active plaques was higher than in caries-free plaques. The results suggest that the glucan production and the gtf gene expression as well as the level of MS and lactobacilli should be higher in the samples of caries-active children than those of caries-free children.