Streptococcus mutans strains were isolated from dental plaques of carious lesions of 53 patients on mitis-salivarius-bacitracin (MSB) and mitis-salivarius(MS) medium as a supplement. The epidemiological investigation was carried out to determine the biotypes and serotypes of S. mutans isolates. For the serotyping, autoclaved extract antigens from the isolates and serotype-specific antisera against seven known serotypes of S. mutans were prepared. The serotypes of the isolates were demonstrated in immunodiffusion test. In addtition, the prevalence of ${eta}$-hemolysis on 5% sheep blood agar plate in restricted anaerobic condition and yellow pigment production on 5% sucrose agar plate in less anaerobic condition among the isolates were investigated. The results were as follows: 1. Forty-eight S. mutans strains were isolated from dental plaque samples of 33 patients (62.3%) among 53 patients. 2. Of the isolates, some strains were not grown on MSB medium. 3. Serotype c S. mutans was found in 60.6%, serotype d was found in 30.3% of the patients who were known to harbor S. mutans. 4. Of. the isolates, serotype c isolates were most prevalent (43.8%), serotype d isolates were 25.0%, and serotype b, e, f and g isolates were also found but in lower frequencies. Serotype a S. mutans were not detected. 5. The correlation between serotype and biotype of the isolates was found in 78.6% of the typing cases. 6. Strains revealed ${eta}$-hemolysis were in 52.1% of the isolates, strains produced yellow pigment were in 47.9% of the isolates, and with one exception, all the strains were belong to serotype c, e and f. 7. The majority of the isolates which revealed ${eta}$-hemolysis appeared to be yellow pigmented, these isolates were belong to serotype c, e and f.