OBJECTIVE Lactate dehydrogenase (LDH) is shown to be an important virulence factor resulting in acid production of Streptococcus mutans (S. mutans), on which the cariogenic potential of S. mutans depends. Differences in cariogenic abilities of S. mutans isolates may be determined by genetic heterogeneity from virulent factors. The relationship between LDH activity or genetic diversity and cariogenicity of S. mutans (serotype c) isolates was studied in this research. METHODS The genome DNA of S. mutans isolates were isolated and LDH gene (ldh) were amplified with specific primers. These isolates came from 34 caries-active individuals and 36 caries-free ones, in which 24 strains showed the high LDH activity and 21 strains showed the low LDH activity. Then genetic diversity of PCR products were analyzed or assessed by restriction fragment-length polymorphism (RFLP). Some of amplified LDH genes from different group were sequenced and assayed. RESULTS It is testified that two genotypes A and B of ldh-RFLP were revealed when LDH genes were digested with Mse I, but Hph I, Mnl I, Dde I, Nla III and Alu I digesting fragments of Idh gene did not show different pattern. Furthermore, Fisher Exact one-Tail Test showed that the proportion of genotype B among strains from caries-free individuals was higher than that from caries-active ones (P = 0.033), while the distribution of genotypes with different LDH activity was different between two groups (two-Tail Test P = 0.017). The sequencing DNA testified that the specific base mutation would lead to multiple kind of genotype resulted. CONCLUSIONS This study indicated that LDH gene of S. mutans clinical strains from different individuals is conservative, while there still is the gene mutation in. The ldh genetic diversity may be related to the low caries sensitivity, and closely correlated with the differences in LDH enzyme activity of S. mutans strains.