The aim of the present study was to characterize bacteria of the genus Streptococcus isolated from the oral cavity of the guinea pig as well as to assess the significance of these microorganisms as potential veterinary and human pathogens. Sixty-two streptococcal isolates recovered from 27 clinically healthy guinea pigs were examined genotypically by sequencing the 16S rRNA and groEL genes. Among these isolates, only 13 could be assigned to a species described previously (mainly Streptococcus parasanguinis, S. mitis and S. suis), and the majority of the remaining ones differed considerably from the streptococcal species known to date (16S rRNA and groEL sequence similarities were < 97% and < 87%, respectively). Based on 16S rRNA sequences, these unidentified isolates were divided into seven groups (clades), of which clades I through III comprised most of the isolates examined and had also the widest distribution among guinea pig colonies. Upon groEL gene sequence analysis, however, members of the three clades grouped together without forming such distinct clusters. The remaining clades distinguished by 16S rRNA sequencing could also be discerned by the second gene, and they contained only a few isolates often restricted to one or a few animal colonies. The present work reveals that the guinea pig mouth is inhabited by a vast number of phylogenetically diverse, so far unrecognized populations of streptococci, most of them being apparently host-specific genomospecies. On the contrary, S. parasanguinis and S. mitis are also common human commensals and S. suis is a well-recognized zoonotic pathogen.