<i>Bacteroides fragilis </i>is frequently recovered from cases of appendicitis with perforation and from infections developing secondary to appendicitis. In order to assess the part played by <i>B. fragilis </i>in the aetiology of appendicitis, quantitative aerobic and anaerobic culture studies of the contents of 49 inflammated appendices were performed. Anaerobic gram-negative non-sporing rods were cultivated from 43 appendices in the range 10<sup>3</sup>–10<sup>9</sup>/g. A total of 1,473 isolates was differentiated by biochemical methods, and 1,374 cultures were found to belong to the saccharolytic species of the genus <i>Bacteroides (B. fragilis, B. thetaiotaomicron, B. vulgatus, B. distasonis </i>etc.).<i> B. fragilis </i>was detected in 31 appendices; the species predominated in 18 samples. <i>B. thetaiotaomicron, </i>recovered from 27 samples, was prevalent in 4 appendices. In one sample, <i>B. fragilis </i>and <i>B. thetaiotaomicron </i>outnumbered the other appendicular bacteria. <i>B. vulgatus </i>was cultivated from 12 appendices, but did not once constitute the prevalent group. It has been previously shown that <i>B. vulgatus </i>(43% of intestinal isolates) and <i>B. thetaiotaomicron </i>predominate in the normal large bowel flora. On the other hand, approximately 80% of pyogenic <i>Bacteroides </i>strains belong to <i>B. fragilis, B. thetaiotaomicron </i>accounting for 19% and <i>B. vulgatus </i>being virtually absent. From these striking differences in species distribution the conclusion was drawn that <i>B. fragilis </i>possesses the highest virulence for man. Species distribution within the 1,374 appendicular isolates of saccharolytic <i>Bacteroides </i>(percentages of 62, 19 and 4.3 for <i>B. fragilis, B. thetaiotaomicron, </i>and <i>B. vulgatus, </i>respectively) was very similar to that encountered in clinical specimens. From the results obtained it becomes evident that pyogenic <i>Bacteroides, </i>in particular <i>B. fragilis, </i>plays an important role in nearly 50% of cases of appendicitis.