The predominant dental plaque flora was compared between two groups of adult beagle dogs, (1) ten with gingivitis and (2) ten with advanced periodontitis. Plaque from a maxillary third premolar was cultured under strict anaerobic conditions. Specimens comprising the marginal periodontal tissues were taken at the plaque sampling site and analyzed for some histological parameters of periodontal disease. The periodontitis dogs scored significantly higher for crevice depth, length of ulcerated crevice epithelium, area of inflamed connective tissue, and loss of attachment. Supragingival periodontitis plaque had significantly higher anaerobic to aerobic ratio, proportions and CFU of esculin negative streptococci, but lower proportions ofActinomyces viscosus. Subgingival periodontitis plaque had significantly higher anaerobic to aerobic ratio, microscopic counts of spirochetes, total viable CFU, proportions and CFU of esculin negative streptococci andFusobacterium nucieatum, as well as CPU ofBacteroides asaccharolyticus, Gram negative bacilli and coccobacilli, but significantly lower proportions ofA. viscosusand unspeciated actinomycetes. The total viable CFU, proportions and CFU of esculin negative streptococci correlated with all four histological parameters of periodontal disease. The CFU ofB. asaccharolyticus, bacilli, and coccobacilli correlated with the length of ulcerated crevice epithelium and loss of attachment, butF. nucleatumonly with the area of inflamed connective tissue and loss of attachment.