Article InformationDental caries is the common infectious diseases of the oral cavity and is caused mainly by oral streptococci. The present study was carried out to investigate the anticariogenic activity of methanol extract of Lagerstroemia speciosa (L.) (Lythraceae) leaves. The inhibitory efficacy of methanol extract was tested against 12 oral isolates of Streptococcus mutans by Agar well diffusion method. The broth cultures of bacteria were swabbed uniformly on sterile Brain heart infusion agar plates and wells of 6mm were punched in the inoculated plates. Standard antibiotic and different concentrations of extract were transferred into labeled wells. Zone of inhibition was measured after incubation. The extract caused a concentration dependent inhibition of cariogenic isolates. Inhibition caused by standard antibiotic was higher than the methanol extract. Preliminary phytochemical analysis showed the presence of saponins, glycosides, tannins and terpenoids. The result of the present study reveals that methanol extract showed significant inhibitory activity against cariogenic isolates. The inhibitory efficacy of extract against cariogenic isolates could be due to the presence of these metabolites. In suitable form, the leaves could be used to treat dental caries.