The in vitro susceptibility of 157 anaerobic bacteria to imipenem was compared with that to cefoxitin, latamoxef (moxalactam) and clindamycin. The isolates were cultured from clinical specimens submitted to Tufts Anaerobic Laboratory between January 1981 and March 1983. Two strains of Bacteroides fragilis only, were highly resistant to imipenem. The incidence of cefoxitin resistance in the Bact. fragilis group has increased at Tufts-New England Medical Center. The data indicate that imipenem is the most active β-lactam drug against anaerobic bacteria, including Bact. fragilis group of organisms. The only difference, since our original studies with thienamycin, is the recognition of two imipenem-resistant strains of Bact. fragilis .