SYNOPSIS Bacteria were isolated from 153 (47-5 %) swabs of the appendix fossa in 322 patients undergoing appendicectomy. The commonest organism was Bacteroides species found in 78 of specimens. Other Gram-negative bacilli such as Klebsiella, or Enterobacter, and Esch. coli were present in 29 and 27 % respectively. Gram-positive cocci were less frequently isolated.A positive culture was obtained more commonly in perforated appendicitis (79 %) than where chronic fibrosis, lymphoid hyperplasia, or acute appendicitis was present or when the appendix was normal. Bacteroides was isolated twice as often in perforated appendicitis.The incidence of wound infection was 19% and varied according to the state of the appendix, being 63 % in perforated appendicitis and 9 5 % where lymphoid hyperplasia was present. Bacteroides was isolated from over 90 % of the wound infections.In the patients with perforated appendicitis where effective chemotherapy was given the incidence of wound infection was 15 % whereas in untreated or inappropriately treated patients it was over 50%.The isolation of bacteroides requires special precautions to be taken both in the collection of the specimen and laboratory culture. It is important that the chemotherapy of postappendicectomy infections include an antibiotic active against bacteroides.Appendicitis is a common condition which frequently requires emergency surgery. Postoperative wound infection has been reported in up to 30% of cases (Lancet, 1971) and this incidence can be influenced by many factors, the most important being surgical skill and technique and the criteria used to define the infection. Bacteria play an important role in appendicitis and the local application of antibiotics (Forbes, 1961;Rickett and Jackson, 1969) or antiseptics (Gilmore, Martin,and Fletcher, 1973) can reduce the incidence of wound infection. Despite the large number of studies into the infective complications of appendicitis, few workers have produced a detailed description of the associated bacteriology, and anaerobic bacteria which account for over 90 % of the bacterial population of the intestine have rarely been reported. The relationship ofpreoperative bacterial flora of the peritoneal cavity surrounding the appendix to the degree of contamination is important in the development of wound infection. There is no agreement about the value of parenteral antibiotic treatment in acute appendicitis which