The anaerobic gram-negative faecal flora of five patients with Crohn's Disease (CD) was identified and compared with that of healthy subjects. For isolation and cultivation of the anaerobic gram-negative rods a non-selective medium was used. There were no significant differences in numbers of Bacteroides and Fusobacterium spp. between patients with CD and healthy subjects. However, the numbers of the "Bacteroides fragilis" group were significantly higher in patients than in controls. The high numbers of the "B. fragilis" group in the faeces of patients were particularly due to B. vulgatus which was 6 times more frequent in patients than in healthy subjects. This indicated that B. vulgatus was responsible for the higher numbers of anaerobic gram-negative rods in the faecal flora of patients with CD.