The minimum inhibitory concentrations of 45 antimicrobial agents were established by the agar dilution method for 37 strains of Bacteroidaceae: Bacteroides fragilis (18 strains), B. thetaiotaomicron (8 strains), Fusobacterium nucleatum (6 strains), and Fusobacterium spp. (5 strains). The fusobacteria tested showed good susceptibility to beta-lactam-antibiotics, tetracyclines, lincomycins, and nitroimidazole compounds. Bacteria of the B. fragilis group were resistant to most of the penicillins and cephalosporins and could not be suppressed with certainty by nitroimidazoles and older tetracyclines. Doxycycline and minocycline, however, showed sufficient activity, and all our Bacteroides strains tested were sensitive to clindamycin. Especially the good in vitro susceptibility of all Bacteroidaceae strains investigated to cefoxitin may offer new therapeutic possibilities.