The minimum inhibitory concentrations were determined in 98 strains of anaerobic bacteria that came from clinical samples against fosfomycin, penicillin, cephalothin, tetracycline, chloramphenicol, clindamycin and lincomycin. The results obtained indicate that fosfomycin is usually inactive against Bacteroides sp. and is active up to 32 µg/ml or less against 85% of Peptococcus and 95% of Peptostrepto-coccus, being consequently comparatively less active than the rest of the antibiotics that were tested against Bacteroides sp., while against Peptococcus and Peptotsrep-tococcus it is less active than penicillin, cephalothin, clindamycin and lincomycin, and somewhat more active than chloramphenicol and tetracycline