Susceptibility to anti-anaerobic agents in the Bacteroides fragilis group varies according to the geographical region studied. In recent years there has been a reduction in the susceptibility of such isolates, especially to certain antibiotics such as clindamycin and cefoxitin. The antimicrobial susceptibilities of 200 strains of the B. fragilis group isolated in 1998 (100) and 2001 (100) from faecal samples of healthy people to anti-anaerobic agents were determined. Meropenem, metronidazole and chloramphenicol showed excellent activity against all isolates. The efficacy of cefoxitin was low, with only 43% of strains isolated in 2001 showing susceptibility. A high prevalence of resistance to clindamycin, reaching 56% of recent isolates, was observed. A significant increase in resistance to new fluoroquinolones, from 0% in 1998 to 12% in 2001, was detected.