A filamentous secretion composed of carboxylated and sulfated acid glycosaminoglycans (AGAG), neutral polysaccharides, and protein(s) appears in the lumen of the Malpighian tubes of the fully grown larvae of the bumblebee,Bombus atratus Franklin. A well-ordered macromolecular array was demonstrated specially for the carboxylated AGAG components of this secretion, based on their linear dichroism and birefringence properties. It is suggested that the carboxylated AGAG macromolecules can acquire a helical conformation when present at the lumen of the organ. The mucous secretion elaborated by the Malpighian tubes ofB. atratus is excreted from the larvae in the form of condensed filaments. Its function remains unclear. Globules with concentric lamination containing protein and neutral polysaccharides were seen detaching from the apical border of the epithelial cells of the Malpighian tubes of the fully grown larvae. It could not be established whether they contribute for the elaboration of the filamentous secretion. Morphologically similar globules have been assumed in some other insect groups to originate from cytolysomes. Urate crystals surrounded by a halo of calcium granules were also found in the lumen of the Malpighian tubes ofB. atratus.