The purpose of this study is to survey the frequency of mutans streptococci species and biotypes isolated from dental plaque in Korean adolescents and adults and the relationship between species of mutans streptococci and DMFT index. The dental plaque samples were collected from the anterior and molar teeth of both of jaws in the 47 human subjects(aged 14 to 29.3 years, average age was 20.2 years). A dental examination was performed for DMFT with the WHO caries diagnostic criteria. The mutans streptococci were cultured selectively on mitis-salivarius bacitracin (MSB) agar plates. The bacterial colonies growing on the MSB plates were then Identified with biochemical tests (for biotyping) as well as polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) targeting dextranase gene (dex) (for the determination of species). The data showed that the prevalence of Streptococcus mutans, S. sobrimus, and S. downei were 93.6%, 12.8%, and 8.5%, respectively. The biotype I (87.4%) and biotype IV (19.1%) were the most frequently detected. The DMFT scores of adolescents and adults harboring both S. mutans and S. sobrinus were significantly higher than those for S. mutans alone (p<0.05). Above results revealed that S. mutans and biotype I were frequently detected in Korean adolescents and adults. In addition, the results suggest that subjects having plaques compared of both S. mutans and S. sobrinus may be more susceptible to caries than those having S. mutans alone.