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Results of antimicrobial susceptibility testing of anaerobes were reviewed for the last 5 years and compared with two previous surveys from this institution. Allowing for differences in methodology, there appears to be a striking increase in penicillin resistance by the "non-fragilis" Bacteroides. Penicillin concentrations of 50 microgram/ml were required to inhibit 70% of 43 strains tested. The clinical implications of this observation are not known, but isolated reports of therapeutic failure when penicillin was used against the non-fragilis Bacteroides have appeared. Penicillin resistance among Clostridium other than C. perfringens was also noted in the current study. Several strains of clindamycin-resistant Bacteroides fragilis have been observed during the last 2 years, and whether this represents a true increase in resistance will require close scrutiny of clindamycin susceptibility in the future. Future surveillance of antimicrobial susceptibility of anaerobic bacteria will be useful in the detection of any major changes in susceptibility profiles. This knowledge will potentially affect the choice of antimicrobial agent in patients with serious infections caused by anaerobes.