Objective To analyze the count of Streptococcus mutans（S.mutans） and Lactobacilli（LB） in different groups and the cases in dental caries. To research the synergistic effect of S.mutans and LB in the process of dental caries. Methods 110 cases with dental caries were selected and divided according to the different degree of caries, nature and ages. To culture bacteria in the selective culture medium, then count the number of colonies. The detection rate of two kinds of bacteria in different groups, means of the bacteria count and the cariogenic cases were analyzed. Results The means of the two bacteria count increased along with the degree of caries increased（P
