To study the hospital epidemiology of Bacteroides fragilis, 343 isolates from infected hospitalised patients (112), infected out-patients (102), and from the faeces of uninfected hospitalised patients (47) and normal subjects in the community (82), were examined by an immunofluorescence technique. In tests with antisera against strains of 20 distinct serotypes of the fragilis group of Bacteroides, 271 (79%) strains were typable. Similarity between strains was estimated by the Jaccard similarity measure and strains were then serogrouped by cluster analysis; 88.1% of hospital strains were typable but only 71.2% of community strains (p less than 0.001). Three serogroups were prevalent among both faecal and infection isolates of hospital strains (p less than 0.01). However, no particular serogroup was prevalent among community strains and no difference was found in the distribution of serogroups between strains from faeces and those from infected lesions. One serogroup showed a significant increase (p less than 0.05) within the period of study. These findings suggest that strains of B. fragilis infecting hospitalised patients may be acquired in hospital and that they may spread to other patients.