Dental caries and periodontal disease are closely associated with dental plaque, the biofilm that results from microbial colonization of the tooth surface. Various experimental models have been described for the initial phase of colonization, including one that utilizes enamel chips positioned within the oral cavity for periods of time up to 24 hours. Scanning electron microscopy of chips removed after 4 hrs in vivo revealed individual bacteria attached to the pellicle-coated surface. Growth of these pioneer organisms resulted in microcolonies that merged to form a spreading monolayer of cocci and rods. Over 80% of the bacteria present in early biofilms were identified as members of four closely related viridans streptococcal species, Streptococcus sanguis, S. oralis, S. mitis and S. gordonii. Veillonella spp, and Actinomyces naeslundii each composed about 5%. Interestingly, veillonellae, actinomyces and streptococci all increased during the first 24 hrs of colonization thereby suggesting important roles for these bacteria in normal daily plaque accumulation [early biofilm development].