Numerous studies on oral biofilms have been performed in vitro, although it is difficult to mimic the oral environment. Here we used an in situ model to conduct a quantitative analysis and comprehensive identification of bacterial communities over time by performing deep sequencing of 16S rRNA genes. We show here that the number of viable bacteria in supragingival biofilms increased in two steps. Using scanning and transmission electron microscopy, as well as confocal laser scanning microscopy, we detected gram-positive cocci during the first 8 h. The biofilm was subsequently covered with a thick matrix-like structure composed of different bacterial morphotypes that diversified as the number of bacteria increased. Streptococcus accounted for 420% of the population until 16 h, and obligate anaerobes such as Fusobacterium, Prevotella and Porphyromonas predominated after 48 h, and this increase was statistically significant after 96 h (Po0.05). Together, our data demonstrate that an initial population of facultative anaerobic bacteria was replaced with a population of gram-negative anaerobic bacteria during oral biofilm formation. This study, therefore, contributes to a comprehensive understanding of the composition of the bacterial microbiota involved in the health of the human oral cavity.