Transfer of imipenem resistance in Bacteroides fragilis was studied. Clinical isolate B. fragilis 10-73 was highly resistant to imipenem. Imipenem resistance was transferred from 10-73 to B. fragilis strain TM4000 at a frequency of 10^-6/input recipient by a filter mating technique. The resistance could also be retransferred. B. fragilis 10-73 and both primary and secondary transcipients produced an imipenem-hydrolyzing metallo-β-lactamase. Acquisition of imipenem resistance correlated with the appearance of plasmid DNA with a size (ca. 13.6kb) similar to that of the donor strain. TM4000 transformed by electroporation with purified DNA of the 13.6-kb plasmid pBFUK1 produced the metallo-β-lactamase and was resistant to imipenem. Transfer was resistant to DNase treatment and no transfer was seen with a sterile filtrate of the donor culture. It is suggested that gene transfer in B. fragilis has the properties of a conjugation system rather than those of transformation or transduction.