OBJECTIVE The antimicrobial susceptibility of anaerobic bacteria isolated from clinical specimens in the referent for Bulgaria anaerobic laboratory was studied in a period of 25 years (1983-2007). METHODS NCCLS-recommended agar dilution methods were used. beta-lactamase activity was determined with nitrocefin discs. RESULTS The 29 antimicrobial agents included in the study were divided according to their in vitro activity against the anaerobic isolates into 4 main groups for guiding empirical treatment: 1st group of metronidazole, chloramphenicol, meropenem, imipenem and combinations of beta-lactam antibiotics with sulbactam--with high activity and drugs of choice for treatment; 2nd group--clindamycin, cefoxitin, carbenicillin/and azlocillin, piperacillin/--with a good activity and low percent of resistant strains; 3rd group--of tetracycline and erythromycin with higher percent of resistant strains including the new macrolides as josamycin, clarithromycin, roxithromycin and azithromycin; 4th group--penicillins/ampicillin, amoxicillin, penicillin/and cephalosporins/cefamandole, cefazolin, cefotaxime and cefoperazone/--not suitable for treatment of infections including Bacteroides fragilis group strains, with a very high percent of resistant strains, probably due to beta-lactamase activity in most of the strains. CONCLUSIONS A continued updating and a follow-up in the changes of antibiotic susceptibility are necessary in every country as resistance patterns vary not only between geographical regions but also even among medical centers and hospitals which may be connected with differences in antibiotic usage in man and animals.