Plasmid pVT745 is a 25.1-kb replicon isolated from Actinobacillus actinomycetemcomitans strain VT745. A previous report described the hybridization of pVT745 in 5 strain-specific patterns to chromosomal DNA from 15 other A. actinomycetemcomitans strains. However, pVT745 does not share homology with the chromosome of the strain from which it was isolated, VT745. It was hypothesized that the shared areas of homology might represent insertion sequence elements and/or transposons possibly encoding resistance to one or more antibiotics. An antibiogram of strain VT745 demonstrated that this strain was uniformly susceptible to all antibiotics examined. Because insertion sequence elements and transposons are mobile genetic elements, a series of cell passaging experiments, followed by Southern hybridization was conducted in a attempt to detect transposition of pVT745 homologous DNA within the chromosomes of several A. actinomycetemcomitans strains. The results of these experiments suggested stability of the homologous DNA both within the chromosome and on the plasmid. It was also possible that pVT745 represented a lysogenic bacteriophage. Phage induction experiments were conducted under conditions that induced a previously described A. actinomycetemcomitans lysogenic phage, but no phage could be induced from strain VT745. Attempts to obtain isolates of VT745 cured of pVT745 were also unsuccessful.