Two Escherichia coli-Bacteroides plasmid-shuttle vectors pNJR5 and pNJR12 were introduced for the first time into Porphyromonas gingivalis W83 by conjugal transfer from E. coli. The transfer frequencies were comparable to those obtained when using colonic Bacteroides as recipients. Both plasmids were maintained in P. gingivalis W83 and could be isolated and introduced back into E. coli. Plasmid DNA extracted from one P. gingivalis W83 pNJR12 transconjugant had an additional 1.5 kb of inserted DNA. Southern-blot analysis of P. gingivalis W83 chromosomal DNA using this inserted DNA as a probe revealed the presence of multiple copies of this sequence on the chromosome. We propose that this DNA represents a P. gingivalis insertion sequence (IS) element and should be referred to as IS1126. This is the first IS element to be isolated from a Gram-negative oral anaerobic bacterium.