Nearly all human oral bacteria exhibit coaggregation, cell-to-cell recognition of genetically distinct cell types. Clumps or coaggregates composed of the two kinds of cells are formed immediately upon mixing two partner cell types. Members of all 18 genera tested exhibit lactose-reversible coaggregation. Many of these interactions appear to be mediated by a lectin on one cell type that interacts with a complementary carbohydrate receptor on the other cell type. A lactose-sensitive adhesin has been isolated from Prevotella loescheii PK1295, and it exhibits the adherence properties observed with whole cells. Other adhesins have been identified and the genes for some of them have been cloned and sequenced. One Streptococcus sanguis adhesin is a lipoprotein that appears to have a dual function of recognizing both a bacterial carbohydrate receptor and a receptor in human saliva. Carbohydrate receptors for some adhesins have been purified from five oral streptococci, and they specifically block the coaggregations with the streptococcal partners that express the complementary adhesins. Coaggregation offers an explanation for the temporally related accretion of dental plaque and bacterial recognition of mucosal surfaces. Early colonizers of the tooth surface coaggregate with each other and late colonizers of the tooth surface coaggregate with each other, but with few exceptions, early colonizers do not recognize late colonizers. Furthermore, bacteria that colonize mucosal surfaces coaggregate with each other, indicating the high degree of specificity of coaggregation in the oral bacterial population.