The adhesive properties of the B. fragilis group to various erythrocytes were studied by haemagglutination technique and compared to encapsulation. Of the 61 strains of Bacteroides groups tested, only 40 strains of B. fragilis exhibited haemagglutination. Treatment of the organisms with carbohydrates and trypsin had no effect on haemagglutination, but boiling for one hour completely destroyed this property. Haemagglutinating activity of B. fragilis was found to be the highest with chick and tanned sheep erythrocytes. The presence of capsule was observed in 46 strains of B. fragilis group. Of the 40 strains of B. fragilis which exhibited agglutination, capsule was found only in 37. The findings of the study suggest a polysaccharide nature for the adhesin. The different patterns of haemagglutination, suggest the involvement of more than one adhesin. Capsule in capsulated strains and fimbriae in non-capsulated strains might be mediating the property of haemagglutination.