The cocoon of Bombus atratus was studied using cytochemical methods and polarization microscopy. The aims were to investigate the components of the cocoon and to establish their stereoarrangement in the cocoon wall. Filamentous layers assumed to be composed of the structural proteins produced by the silk glands appear to make up the framework of the whole structure. They are arranged nearly parallel to the cocoon's surface and depict some reorientation of their protein macromolecules when compared with the situation found in the lumen of the silk glands. These layers enclose masses of pollen grain remnants excreted by the larvae as it spun the cocoon. The pollen grains are stuck together by a mucus which has the cytochemical and some of the anisotropic properties of the secretion discharged by the Malpighian tubules of the larva. Mucous layers, assumed to be produced by the silk glands, cover the cocoon externally. Eventually they also appear associated with the inner filamentous layers. Urate crystal units, sometimes containing acid glycosaminoglycans in their cores, occur in the cocoon wall and come from the Malpighian tubules. The cocoon of B. atratus is not impregnated with calcium Ver menos