Crease of Streptococcus salivarius is believed to play a critical role in bacterial ecology and pH homeostasis in the mouth, and consequently affect the pathogenesis of dental caries and periodontal diseases. Expression of the urease gene is greatly enhanced by low p. f. excess of Carbohydrate, and faster growth. It was observed that urease activity of the strains of S. salivarius that exhibited no of low urease activity was not increased even in low pH condition. In this study, it was hypothesized that the urease gene of the strains is absent, defected, or greatly changed by genetic combination. In order to prove this hypothesis, chromosomes were obtained from 28 S. salivarius strains which had been isolated from normal teeth and carious lesions, subjected to polymerase chain reaction (PCR) using primers encoding highly conserved sequence from ureC, and then the obtained PCR products were compared. The results were as follows: 1. After PCR the strains generated either one of 0.54- and 1.3-kbp PCR products, or none. 2. All 16 strains having a higher urease activity(<50${mu}mol/min/mg$) produced 0.54-kbp PCR products. 3. Twelve strains without urease activity and with a lower urease activity(<50${mu}mol/min/mg$) yield either one of 0.54 and 1.3-kbp PCR products, or none. 4. The DNA sequence of the 0.54-kbp PCR product (pCAP-0.54) exhibited 95% identity to the ureC of S. salivarus 57.I; 30bp were found to be different, which led to difference of only 2 amino acids in the sequence. 5. The DNA sequence of the 1.3-kbp PCR product(pCAP-1.3) was found to be highly homologous to the aminopeptidase C gene of Streptococcus thermophilus. Overall results indicate that there are considerable variations of the urease genes from S. salivarus strains and the variations may affect the uncolytic activity of the bacteria directly of indirectly.