The ultrastructural locations of the group-specific polysaccharide and the type-specific polysaccharides Ia, Ib, II and III of group B streptococci (Streptococcus agalactiae) were studied on isolated walls by the direct immunoferritin technique. The type polysaccharides were located exclusively on the outer side of the wall on which they formed a distinct capsule. Except for strain 58/59 (type Ia) the thickness of the capsule was characteristic of each strain investigated. In all strains the type-specific ferritin labelling was confined to the outer surface of the capsule. The group-specific polysaccharide could be demonstrated on the inner surface in all strains tested. It could also be demonstrated on the outer surface in strains 59/59 (type Ib) and 8/66 (group B variant) and on most of the walls of strain 58/59 (type Ia). The failure to detect this antigen on the outer side of the walls of strains 60/59 (type II) and 13/63 (type III) and on some walls of strain 58/59 was probably due to the thickness of the type polysaccharide capsule.