Imipenem is a highly active drug against the Bacteroides fragilis group of organisms. On the basis of a nationwide survey of over 500 isolates, it was found that the frequency of imipenem resistance was less than 0.1%. We have a highly resistant Bacteroides distasonis isolate, TAL7860, for which the following MICs (micrograms per milliliter) were determined by agar dilution: cefoxitin, greater than 128; moxalactam, greater than 128; piperacillin, greater than 128; imipenem, 16; and SCH34343, 16. Resistance was shown to involve both a beta-lactamase and an outer membrane permeability barrier. beta-Lactamase kinetics studies with several beta-lactams, including imipenem, revealed similar hydrolytic efficiency in comparison with those found for the B. fragilis strains. An imipenem outer membrane permeability barrier was detected for TAL7860, which was approximately sixfold more effective for B. fragilis TAL3636 and TAL2480. Significant inhibition of nitrocefin destruction was also shown with sulbactam and clavulanic acid at 10 mumol and dithiothreitol at 10 mM. No inhibition was seen with 10 mM EDTA. Differences in physicochemical properties and inhibition studies suggest that this beta-lactamase is different from the imipenem-inactivating metallo-beta-lactamase previously described in B. fragilis. We demonstrated a significant permeability barrier to clavulanic acid and sulbactam, which resulted in loss of synergism between these clinically employed beta-lactamase inhibitors and beta-lactam drugs. The novel beta-lactamase activity in conjunction with a limited permeability in TAL7860 resulted in resistance to all commonly employed beta-lactams, including the newest and most potent beta-lactam drugs.