Streptococcus sanguis comprises a heterogeneous group of oral streptococci indigenous to the oral cavity of humans. A total of 289 isolates from an infant population (n=37) were tentatively identified as S. sanguis on the basis of the distinctive colony morphology as shown on MM10-sucrose non-selective medium. These isolates were divided into four biovars of S. sanguis as determined by an extended panel of biochemical attributes. Chromosomal DNA was extracted from each isolate, and an AP-PCR fingerprint profile was obtained to allow study of the diversity within and among the infants. In this study, all four biovars of S. sanguis were detected in the infants. A wide genotypic diversity of S. sanguis was observed among these isolates; on average, each infant harbored 2.7 unique amplitypes as shown by the AP-PCR fingerprints. To explore the phylogenic relationship among these S. sanguis isolates, 20 strains representing the four biovars were selected at random for sequencing of their 16S rDNA and 16S-23S rDNA intergenic spacer chromosomal loci. Two major sequence patterns were identified within the 16S rDNA sequences. A phylogenic analysis showed that members from each of the four biovars of S. sanguis bore close relationship with the type-strain ATCC 10556 sequence, and that all of the isolates representing the four biovars could be clustered into two main phylotypes. The biovars were distributed throughout the phylotypes, indicating no correlation between the genetic and phenotypic groupings.