Objectives: The aims of this study were to investigate the relationships among salivary levels of mutans streptococci, debris index and caries experience of schoolchildren in Kanchanaburi province. Methods: This cross-sectional study was performed on schoolchildren studying in a school near Thai-Myanmar border. Oral health status examination was carried out according to WHO criteria (1997). Stimulated saliva was collected and analysed for mutans streptococci by micromethod using MSB agar as selective medium. The number of mutans streptococci was transformed to log cfu/ml saliva for analysis. Oral examination for debris index (DI) was recorded by Greene and Vermillion method. Results: 355 schoolchildren, 6-13 years old, 185 males and 170 females were recorded. The prevalence of mutans streptococci infection was 96.5% in this study group. The mean log cfu/ml of MS count and oral cleanliness were 5.066 and 0.923 respectively. The mean values of caries experience in deciduous and permanent dentition, dmft and DMFT were 3.389 and 1.994 respectively. The Spearman rank correlations showed weak significant correlation (p<0.05) between MS count and deciduous caries. To identify the differences in debris index and dental caries indices among the three salivary levels of mutans streptococci, the Kruskal-Wallis one way ANOVA on ranks showed statistical significance (p<0.05) in d and dmft, while debris index did not show any. Multiple comparison procedures showed significant differences (p<0.05) in d and dmft values between low and high and between medium and high MS count groups. Conclusions: Schoolchildren with higher salivary MS level had higher number of deciduous caries. This study was supported by Mahidol University Research Grant.