An improved understanding of the mechanisms of resistance and transfer in Bacteroides fragilis has been gained over the past decade. B. fragilis resistance to most penicillins is largely due to production of chromosomal beta-lactamases, although recent findings indicate these micro-organisms can acquire novel beta-lactamases, which can even inactivate imipenem. Several of the beta-lactamases of B. fragilis are transferable. Most of our understanding of transferable antimicrobial resistance in Bacteroides spp. has been gained through studies of the clindamycin-erythromycin resistance determinant found on pBFTM10. This resistance is encoded on a transposon and is widely distributed among the naturally occurring clindamycin-resistant isolates. Recent studies have shown that DNA can be transferred from Escherichia coli to B. fragilis, from one B. fragilis to another, and from B. fragilis back to E. coli. Data from a large survey in the United States indicate that susceptibility patterns of B. fragilis differ in each of the eight participating centres. Overall, piperacillin and cefoxitin have been found to be the most active beta-lactam agents.