Post-antibiotic effect (PAE) is the transient suppression of bacterial growth after brief antimicrobial exposure. While numerous reports have described PAE with aerobic and facultative microorganisms, virtually no studies have been conducted with anaerobic isolates. Intraabdominal isolates of the Bacteroides fragilis group were exposed for one hour to antibiotic (cefoxitin, cefotetan, and imipenem) concentrations two to four times the minimal inhibitory concentration (MIC). Post-antibiotic effect was described as the difference between the time required for microbial growth in the test versus the control to increase one Log10 above the quantitation observed immediately after drug removal. Bacteroides fragilis, B. ovatus, B. thetaiotaomicron and B. vulgatus exhibit PAEs for all test compounds. The time intervals for the PAEs were strain variable and ranged from six to 50 hours. No PAE was demonstrated with B. distasonis strains by the broth dilution technique. The results suggest that brief high dose exposure of some members of the B. fragilis group to anaerobe active beta-lactams produces a prolonged suppression in growth. In theory, a prolonged PAE could influence the dosage regimentation of selective antibiotics.