Various investigations have been carried out to elucidate the causative relationship between specific oral bacterial species and dental caries since it was first demonstrated that selected streptococoal species produced dental caries in germfree rats when fed a high-sucrose diet. Now, S. mutans is considered to play an important role in the development of dental caries in animals and humans, and only a limited number of species of bacteria other than S. mutans are occasionally found to be cariogenic in experimental animals. In this regard, association of the number of S. mutans in approximal plaque with caries experience (DMFT) was studied from 137 Korean school children($10{sim}11$ year old). Biotypes of the collected strains of S. mutans were determined, and their relationship with caries status was also examined. The following results were emerged from the study. 1. S. mutans was detected in the plaques of all children. 2. Statistically significant positive correlation(r=0.445, p<0.001) was found between the caries experience(DMFT) and the number of S. mutans in approximal plaques. 3. The number of S. mutans were significantly higher(p<0.001) in plaques removed from carious surface than from sound surface. 4. The most frequent biotype was biotype I(78.8%), followed by IV(33.1%) and V(09.5%). Biotype II was isolated in the plaque of two children(1.7%) only. 5. There was no apparent relation of specific biotypes to carious status.