Salivary amylase is known to bind specifically to several species of oral streptococci. To assess the importance of this interaction in bacterial colonization of the oral cavity, we determined the proportion and identity of amylase-binding bacteria (ABB) in dental plaque of humans and various salivary amylase-secreting and non-secreting mammalian species. The numbers of ABB in undisturbed plaque collected over time from tooth surfaces of six human volunteers or from 14 other mammalian species were determined by means of a replicating assay. The mean proportion of ABB cultured aerobically from human teeth at 2 h was 10.5% (SD 10), at 8 h 7.9% (8), at 24 h 13% (11), and at 48 h 12% (9). The mean proportion of anaerobically cultured ABB found at 2 h was 3% (SD 4), at 8 h 5% (5), at 24 h 12% (9), and at 48 h 16% (12). Amylase-binding bacteria cultured from these samples resembled Streptococcus mitis, Streptococcus gordonii, Streptococcus salivarius, Streptococcus crista, or unidentified streptococci. In addition, only animals exhibiting salivary amylase activity in their saliva harbored ABB (ranging from 2 to 31% of the total flora), with the exception of the pig, where no ABB were found to colonize, despite considerable amylase activity in saliva. Only strains resembling S. mitis and S. salivarius and unspeciated strains were isolated from these mammals. These results suggest that amylase-binding streptococci are the predominant ABB in human plaque, and their numbers generally increase as plaque develops. Since ABB colonized only the oral cavities of hosts demonstrating salivary amylase activity, the ability to bind amylase may play an important role in oral colonization by these bacteria.