SUMMARY: The outer-membrane complexes from 40 strains of Bacteroides, representing eight of the species included in the Bacteroides fragilis group, were released by EDTA treatment. The component polypeptides were examined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis on slab gels. Within a species (biotype) the patterns produced indicated marked similarities in the structures of the surface proteins among the strains examined. The patterns produced by strains belonging to different species, however, showed fewer similarities. An unknown organism could therefore be identified to species level using this technique and a few selected biochemical tests.