Objectives: To investigate the formation of oral biofilm on various dental ceramics in vivo. Method and Materials: Five different ceramic materials were included: a veneering glass- ceramic, a lithium disilicate glass-ceramic, a yttrium-stabilized zirconia (Y-TZP), a hot isostatically pressed (HIP) Y-TZP ceramic, and an HIP Y-TZP ceramic with 25% alumina. Test specimens were attached to individually designed acrylic appliances; five volunteers wore these appliances for 24 hours in the maxillary arch. After intraoral exposure, the samples were removed from the appliances and the adhering biofilms vitally stained. Then, the twodimensional surface coating and thickness of the adhering biofilm were determined by confocal laser scanning microscopy. Statistical analysis was performed using one-way ANOVA with the level of significance set at .05. Results: Significant differences (P .001) in the bacterial surface coating and in the thickness of the biofilm were found between the various ceramic materials. The lowest surface coating (19.0%) and biofilm thickness (1.9 µm) were determined on the HIP Y-TZP ceramic; the highest mean values were identified with the lithium disilicate glass-ceramic (46.8%, 12.6 µm). Conclusion: Biofilm formation on various types of dental ceramics differed significantly; in particular, zirconia exhibited low plaque accumulation. In addition to its high strength, low plaque accumulation makes zirconia a promising material for various indications (including implant abutments and telescopic crowns) that previously were met only with metal-based materials. Keywords: bacteria, biofilm, dental ceramics, in vivo, zirconia