We have recently isolated several groups of bacteriophages infecting Actinobacillus actinomycetemcomitans from periodontal lesions in patients with rapidly destructive periodontitis. Bacteriophage infection of these bacteria in these patients was restricted to periodontal pockets showing radiographic evidence of recent bone loss and suggests an association between phage-infected A. actinomycetemcomitans and active periodontal disease. On the basis of the biological activity of bacteriophages we propose a working hypothesis to explain the mechanism by which a phage may increase bacterial virulence in periodontal disease.