Abstract:The monthly changes in the counts of anaerobic bacteria, streptococci, and mutans streptococci in plaque of caries-susceptible rats (SDC) and control rats (CSC) were examined at 1, 2, 3, and 4 months. Carious lesions were also evaluated at each change by Keyes' method .No changes in the bacterial counts were observed but there was a transient increase at 3 months , especially in mutans streptococci counts. Also, no significant difference in bacterial counts was found between SDC and CSC rats. However, SDC rats had significantly higher caries scores than CSC rats after 3 months (p<0.01). The monthly age of SDC rats when their caries scores were increased was consistent with the age when their bacterial counts were significantly increased.These results suggested that the number of mutans streptococci contributed to dental caries in SDC rats, but that it was only a necessary conditions, but not a sufficient condition, for the occurrence of caries.