Actinobacillus actinomycetemcomitans is a suspected etiologic agent in destructive periodontal diseases. The detection of bacteriophages in A. actinomycetemcomitans in the subgingival plaque of patients with rapidly destructive forms of periodontitis led to the hypothesis that bacteriophage infection might increase the virulence of this bacterium (19). A. actinomycetemcomitans was isolated from 68 subjects from the Netherlands and Switzerland with localized juvenile periodontitis, rapidly progressing periodontitis, or adult periodontitis, and was tested for the presence of temperate bacteriophage with the overlay plate technique. More than half of the A. actinomycetemcomitans strains were found to release bacteriophage which formed individual plaques on indicator strains. Electron microscopy of preparations from 7 strains revealed virions with an icosahedral head and a contractile tail typical for double-stranded DNA bacteriophages. The presence of A. actinomycetemcomitans carrying temperate bacteriophage was not correlated with the composition of the subgingival microflora nor with the clinical form of periodontal disease. Destructive periodontal disease of subjects with phage-carrying A. actinomycetemcomitans was not more severe than of subjects with phage-free A. actinomycetemcomitans as determined by several clinical parameters. In contrast, the pocket depth and the attachment loss were significantly lower for adult periodontitis subjects with phage-carrying A. actinomycetemcomitans. It seems unlikely that the frequently occurring temperate bacteriophages increase significantly the virulence of A. actinomycetemcomitans.