SUMMARYRoots of leek, colonized or not by Glomus clarum Nicolson & Schenck, were ultrastructurally examined for the presence of difTerent sugars using lectin-gold complexes. A^-acetyl-D-glucosamine (GlcNAc), A^-acetyl-Dgalactosamine (GalNAc), i,-fucose (L-FUC), D-mannose (D-Man) and sialic acid sugar residues were localized in various cellular components of the mycorrhizal fungus. GlcNAc, GalNAc, D-Man and sialic acid residues were localized in the colonized host, particularly in the plasmalemma and cytoplasm surrounding the arbuscular hyphae. The same sugars were not present in the non-colonized host. These results indicated that the production of these sugars had been triggered during the plant-fungus association. In the interfacial matrix, GalNAc and DGalactose (D-Gal) residues were present in moderate amounts. In cell walls and protoplasm of non-colonized and colonized hosts, mainly D-Gal was detected. Ribonucleic acid (RNA) was cytochemically localized in the fungus and host by means of a RNAse-gold complex. Of interest, was the specific localization of RNA in the granular wall layer of vesicles. The ultracytochemical results are discussed in relation to host-symbiont interaction, along with speculations on the function and significance of these macromolecules.