The role of bacteriain the pathogenesis of periodontal disease is controversial. Although bacterial penetration has been demonstrated in human Periodontitis, relationships of bacteria to varying levels of periodontal health in other animals has not been well documented. In this study of beagle dog periodontium, bacteria were counted within the various tissue regions of sites which exhibited periodontal health, gingivitis, or Periodontitis.Affected sites within whole block sections from two aged beagle dogs were prepared for investigation by light (LM) and scanning electron microscopy (SEM), and bacterial counts were subjected to statistical analysis for differences.Periodontitis sites viewed with LM had more bacteria than healthy sites. Neither LM nor SEM observations yielded any other differences in bacterial counts between the levels of periodontal health. When viewed using SEM, the sulcular epithelium had more bacteria than the other tissue regions in diseased sites. Relative to the number of bacteria in plaque, however, counts from all microscopic observations were very low.These findings indicate that the presence of bacteria within the periodontium is of questionable significance in the pathogenesis of spontaneous periodontal disease in beagle dogs.