Cefoxitin resistance, but not resistance to clindamycin, erythromycin, lincomycin, or tetracycline, was transferred by a conjugation-like process from Bacteroides thetaiotaomicron UN101, a clinical isolate harboring four kinds of plasmids, to other Bacteroides species. Of a sample of 20 cefoxitin-resistant transconjugants, 8 contained all 4 plasmids, 10 contained 1 to 3 plasmids, and 2 contained no plasmids.Although the transfer of resistance to several antibiotics has recently been demonstrated in Bacteroides species (6,7,9,12), there have been no reports of transferable resistance to P-lactam antibiotics in this genus. In this paper, we report on a conjugation-like transfer of cefoxitin resistance from a strain of B. thetaiotaomicron to other species of Bacteroides.Four strains of Bacteroides were used: B. thetaiotaomicron UN101, a multiply antibioticresistant isolate from a blood culture at the University of Nebraska Hospital; B. fragilis 638 rfm, a rifampin-(6) and chloramphenicol-resistant isolate; B. fragilis V479-1, harboring plasmid pBF4, which determines resistance to clindamycin (13); and B. uniformis V528, a rifampinresistant isolate (13). Routine growth of bacteria has been previously described (9).Minimal inhibitory concentrations of antibiotics were determined essentially by the agar dilution method for anaerobic bacteria (11), except that the plates were incubated in an anaerobic chamber instead of anaerobic jars.Four methods (2, 4, 5, 9) were used to isolate and detect plasmids. Analysis of plasmids was by agarose gel electrophoresis as described previously (9).Curing experiments were performed by diluting overnight cultures 1:20 in fresh supplemented brain heart infusion broth containing 5 ,uM ethidium bromide, incubating for 24 h anaerobically at 37°C, and plating dilutions on an antibiotic-free medium. Individual colonies were then tested for antibiotic resistance. Transferability of antibiotic resistance markers was tested by the filter-mating technique as described previ--ously (9).