As a consequence of antibiotic consumption, the entire microbial ecosystem attached to man is evolving towards resistance. In Spain, penicillin resistance (MIC > 0.5 mg/l) is found in about 10 % of Peptostreptococcus, Clostridium perfringens and Eubacterium, and in 50 % of Veillonella. Cefoxitin resistance (> 2 mg/l) is present in 10 to 20 % of Peptostreptococcus and Clostridium, and in 50 % of Eubacterium; 21 % of Bacteroides (fragilis group) strains are resistant to 16 mg/l. A very low rate of imipenem resistance (greater-than-or-equal-to 128 mg/l) is found among Bacteroides (1 %), but for 5 % of the isolates MICs of imipenem are 2 to 4 mg/l. Metronidazole resistance (> 8 mg/l) is found in 5 to 10 % of Peptostreptococcus, Clostridium and Veillonella, and in less than 1 % of the Bacteroides fragilis group. Erythromycin resistance (> 2 mg/l) is present in over two-thirds of the Peptostreptococcus, Veillonella and Fusobacterium isolates, and in 27 % of the Clostridium perfringens strains. Clindamycin resistance (> 4 mg/l) is found in 10 to 20 % of the Peptostreptococcus, Clostridium and Eubacterium isolates, and in 20 % of the Bacteroides of the fragilis group, this rate being higher (30 %) among faecal isolates.