The authors isolated in common surgical practice 83 anaerobic bacteria mainly bacteroides and clostridia. They analyse their pathogenic role in relation to the site of sampling. Comparing the bacteriological results with a clinical study of 96 cases of generalised peritonitis, there appeared to be no significant differente between the prognosis in peritonitis and anaerobic bacteria and those without anaerobic bacteria. The mortality was identical in septicemia due to anaerobic and aerobic bacteria; jaundice was just as frequent in both series. However, the prognosis of peritonitis due to anaerobic bacteria seems better when antibiotic treatment is adapted to the bacteria, e.g. lincomycin, metronidazole. With regard to anaerobic bacteria, which represent only part of the fecal flora, it may be dangerous by selection of resistant strains to use prophylactic antibiotics as a routine; local treatment of the septic focus seems the most important. The pathogenic role of anaerobic bacteria in digestive pathology is far from clearly defined; one should not follow the fashion of using even specific antibiotics without justification.