Streptococci are the primary component of the multispecies oral biofilm known as supragingival dental plaque; they grow by fermentation of sugars to organic acids, e.g., lactic acid. Veillonellae, a ubiquitous component of early plaque, are unable to use sugars; they ferment organic acids, such as lactate, to a mixture of shorter-chain-length acids, CO(2), and hydrogen. Certain veillonellae bind to (coaggregate with) streptococci in vitro. We show that, between 4 and 8 hours into plaque development, the dominant strains of Veillonella change in their phenotypic characteristics (coaggregation and antibody reactivity) as well as in their genotypic characteristics (16S RNA gene sequences as well as strain level fingerprint patterns). This succession is coordinated with the development of mixed-species bacterial colonies. Changes in community structure can occur very rapidly in natural biofilm development, and we suggest that this process may influence evolution within this ecosystem.