Antimicrobial susceptibility testing of anaerobic bacteria has assumed greater importance in recent years becauseof (1) an increasein recognition of the clinical significance of anaerobes, (2) a decreasein predictabilityof susceptibility patterns, (3) the appearance of new antimicrobial agents with variable activity against anaerobes, (4) an increase in use of and demand for susceptibility data by clinicians, and (5) the availability of a standardized reference and practical methods for susceptibility testing of anaerobes. The broth microdilutionand broth disk-elutiontechniquesare suitablefor clinical microbiology laboratories and have been shown to provide results comparable to those obtained with the reference agar-dilution method developed by the National Committee for Clinical Laboratory Standards. The ability to produce β-Iactamase has been demonstrated in most strains of the Bacteroides fragilis group and in over half of other Bacteroides. Increasing resistance of B. fragilis to clindamycin has been reported, and the newer β-Iactam antibiotics, including moxalactam, piperacillin, cefoperazone, and cefotaxime, have variable activity against this organism. Most medical centers should perform susceptibility tests on anaerobes isolated from patients with bacteremia and bone and joint and central nervous systeminfectionsand on those anaerobes isolated in pure culture from other specimens and also should test anaerobic isolateson special request of clinicians.