Streptococcus mutans SpaP mediates the binding of this cariogenic bacteria to tooth surfaces. It was reported that the SpaP of S. mutans clinical isolates could be classified to 2 genotypes, type A and B. Our aims are to examine spaP genotypes in often-used S. mutans laboratory strains as well as clinical isolates and to explore the relationship between the genotypes of S. mutans strains and their adherence to salivary-agglutinin (SAG). The sequences of SpaP of 11 S. mutans strains were analyzed with alignment tools. Out of these strains, 9 strains were examined for their adherence to SAG-coated surfaces. The SpaP expression on the cell surfaces and in the spent media of 9 strains were examined by a dot-blot assay. Based on the alignment of the variable V region of SpaP, 9 strains were classified as previously-defined type-A and 3 strains type-B. Among type-B strains, the SpaPs of GS5 and HG723 contain a premature stop codon which resulted in loss of adherence and absence of SpaP expression on the cell surfaces. However, clear SpaP expression was observed in the spent media of both strains. The type-B strain UA159 demonstrated low SpaP expression on the cell surface, but it showed similar adherence ability as the type-A strains. In conclusion, the presence of SpaP on the cell surface determines the adherence of S. mutans to SAG. No difference in SAG-mediated adherence could be seen between type A and B strains, probably due to the limited number of type B strain tested.