This study was designed to investigate, using 6 beagle dogs, the levels of selected putative pathogens in healthy sites, in gingivitis sites, and in sites with histologically confirmed attachment loss. Levels of attachment loss increased with increasing periods of ligation and reached a maximum of 0.15 mm at 57 days. Both histological attachment level and histological pocket depth were found to vary significantly with health/disease status (p < 0.0001). Higher numbers of total colony-forming units were seen for ligated sites than for healthy and gingivitis sites. Levels of Porphyromonas gingivalis and Prevotella intermedia also changed significantly with health/disease status (p < 0.001). These organisms showed their greatest increases at the time of the most intense attachment loss. Higher levels of Fusobacterium nucleatum were seen in the gingivitis sites than in healthy or ligated sites. Low levels of Campylobacter rectus and Capnocytophaga spp. were detected throughout. The morphometric microbiological analysis revealed unexpectedly high %s of motile rods, while spirochetes were found in very low %s. The total number of bacterial cells detected using phase contrast microscopy was not found to vary significantly. None of the morphotypes were demonstrated as showing significant changes with health/disease status.