Results of antimicrobial susceptibility testing of anaerobic bacteria isolated at the Mayo Clinic were reviewed for 1982 through 1987 and compared with a previous survey during 1977 through 1981 at this institution. Between the earlier and the later period, clindamycin resistance increased in the Bacteroides fragilis group (from 4% of isolates to 8%). We noted continuing penicillin resistance among Bacteroides species other than B. fragilis and rare penicillin resistance among Fusobacterium organisms, with four isolates during the 1982 through 1987 period being beta-lactamase producers. The high levels of resistance to some agents seen in certain Clostridium species in 1977 through 1981 were not as great in the current survey. No major changes were noted in the susceptibilities of C. perfringens, anaerobic non-spore-forming gram-positive bacilli, and anaerobic gram-positive cocci.