A detailed bacteriologic study was done on 161 patients operated for appendicitis. Aerobic and anaerobic cultures were taken from the blood, the appendicular lumen, mucosa, serosa, fossa, and from the wound after closure of the peritoneum. There is no correlation between the degree of appendicitis and the incidence of positive blood culture. The infection spread through the appendicular wall as the disease progressed. Aerobic infection was common in early appendicitis but a mixed aerobic and anaerobic infection was predominant in late cases. Late appendicitis, a positive wound culture at the end of the operation, the duration of symptoms of over 36 hours before operation and the age of the patient over 50 years were all associated with an increased incidence of septic complication. From the antibiotic sensitivity on the bacteria isolated, the most effective agent against anaerobes was metronidazole. Effective agents against the aerobes were aminoglycosides and cephalosporins. The best single agent against both anaerobes and aerobes was moxalactum.