The purpose of this study was to analyse quantitatively the early bacterial plaque formed on resin composite and human enamel in vivo, using a confocal laser scanning microscope. Test pieces of resin composite and human enamel were retained at the buccal surfaces of the upper first molars of three volunteers for 4, 8 and 24 h to allow plaque formation. Then, the specimens were immersed in propidium iodide in phosphate-buffered saline to stain adherent bacteria and observed with a confocal laser scanning microscope. The ratios of the area occupied by microorganisms to the whole area of the optical field were calculated using a photo-image analysis system. The thickness of the plaque was also measured. Quantitative analysis revealed that the resin composite showed significantly higher bacterial adherence than human enamel throughout the test period. A difference was noticed in the morphology of the bacteria between the two groups. Our findings suggest that resin composite shows higher bacteria adherence during early plaque formation compared with human enamel. In addition, the present findings may suggest a presence of the difference in bacterial composition of plaque in both specimens.