A study was undertaken in Japan to evaluate the susceptibility patterns of Bacteroides fragilis group species (849 strains) isolated from December 1986 through May 1991. All of the strains, which included B. fragilis (610 strains), Bacteroides thetaiotaomicron (201 strains), and Bacteroides distasonis (38 strains), were studied for susceptibility to imipenem and 16 other antimicrobial agents by an agar dilution method. Metronidazole was the most active agent; the minimal concentration for 90% inhibition (MIC90) was 0.78 micrograms/mL, and no isolate was noted to be resistant to it during the entire study period. Amongst the beta-lactam agents tested, imipenem was the most effective antimicrobial drug; the rate of resistance to this agent was only 3.3%. Cefoperazone/sulbactam (MIC90, 6.25 micrograms/mL) and cephamycins (MIC90, 25-50 micrograms/mL) were found to be more active against B. fragilis strains, whereas minocycline (MIC90, 6.25 micrograms/mL) showed better activity against B. thetaiotaomicron and B. distasonis strains. Increasing resistance to imipenem was observed in B. fragilis (2.0%-5.9%) and B. thetaiotaomicron (2.5%-6.1%) during the 4-year study period. Detailed investigation of beta-lactamases from imipenem-resistant strains demonstrated that, amongst them, six of the B. fragilis strains for which the MICs of imipenem were > or = 25 micrograms/mL were producing imipenem-hydrolyzing metallo-beta-lactamase.