In vitro susceptibility data were collected for co-amoxiclav and other antimicrobial agents against 1297 recent anaerobe isolates collected in Europe and the USA. The co-amoxiclav (amoxicillin/clavulanic acid) MIC(50/90)s (amoxicillin/clavulanic acid concentration in a ratio of 2:1, expressed in terms of amoxicillin concentration in mg/L) were 0.5/4 for Bacteroides fragilis, </=0.125/1 for Prevotella species, </=0.125/0.25 for Fusobacterium nucleatum, 0.5/1 for Eikenella corrodens, 0.25/8 for Peptostreptococcus anaerobius, </=0.125/0.5 for Micromonas (Peptostreptococcus) micros, </=0.25/0.5 for Fingoldia (Peptostreptococcus) magna, and </=0.125/0.125 for Porphyromonas species. The co-amoxiclav susceptibility rate for B. fragilis was 94.6%, for P. anaerobius 84.3% and for all other species tested 100%. These data indicate that co-amoxiclav remains an effective drug for the antimicrobial treatment and prophylaxis of many anaerobic infections. Among the comparator drugs, metronidazole was very active against all bacterial species (>96% susceptible) except E. corrodens (MIC(50/90) of >32/>64 mg/L), which is a capnophilic organism. Imipenem was also highly active against all species (>98% susceptible). Levofloxacin and clindamycin were the least potent agents tested, particularly against Bacteroides, Prevotella and Peptostreptococcus (levofloxacin susceptibility rates: Bacteroides 72.7%, Prevotella 71.5%, F. magna 72.4%; clindamycin susceptibility rates: Bacteroides 79.5%, Prevotella 92.1%, F. magna 84.7%).