During 1994, the in vitro antibiotic susceptibility of 306 anaerobic bacteria was performed in 4 hospitals, by the reference agar dilution method. Among the 129 B. fragilis group strains, only two B. fragilis strains were resistant to the three carbapenems and all beta-lactams, even combined with beta-lactamase-inhibitors while metronidazole resistance could not be detected. Evolution in antibiotic resistance rates could be assessed only for piperacillin whose resistance rates increased to 20%. beta-lactamase production was detected respectively for 27% of Prevotella and 17% of Fusobacterium strains. No beta-lactamase activity was seen among Gram positive anaerobes. On the whole anaerobic strains resistance rates were: biapenem, imipenem, meropenem and piperacillin-tazobactam 0.7, amoxicillin-clavulanic acid or metronidazole 2, piperacillin 11.3, amoxicillin 31%, respectively. The three carbapenems demonstrated a good in vitro activity against most anaerobes with few differences between them.