The minimum inhibitory concentrations (MIC) of nalidixic acid were determined for thirty-four strains of Clostridium sp. by an agar dilution technique. The MIC range of Clostridium perfringens ran from 0.03 microgram/ml to 64 microgram/ml, according to a bimodal distribution. One half of the strains was inhibited by 0.25 microgram/ml. 64 microgram/ml was able to inhibit all the strains tested. The author suggests a systematic utilization of media with and without 40 microgram/ml nalidixic acid. These two media were parallely inoculated with clinical specimens where anaerobic bacteria were suspected, and allowed the isolation of gram negative anaerobes.