The mode of appearance and assembly of cyst wall filaments on the surface of Giardia duodenalis trophozoites committed to encyst was analysed by scanning and transmission electron microscopy (SEM and TEM) and by fluorescence microscopy (FM). SEM showed a progressive appearance of fibril patches, predominantly on the anterior area of ventral and dorsal surfaces, which then spread and coalesced. By TEM, ruthenium red (RR) displayed staining in encysting cells as rodlike spots of variable diameter (3–25 nm), possibly microfibril tips with polyanionic moieties, that displayed tangential associations and random orientations over the cell membrane. In FM assays, the 1,10-phenanthroline derivative of ruthenium red (RR/oPHE) was a specific ligand for these assembling fibrils and this staining was significantly blocked by N-acetylgalactosamine (GalNac) and galactosamine (GalN). Interestingly, RR staining was lost when the cyst wall was completely assembled and thickened as observed by TEM and FM. Kinetic FM assays, in which a mAb specific for a 26 kDa Giardia cyst wall polypeptide was used concomitantly with RR/oPHE staining, showed a differential pattern for the appearance and reactivity of polypeptide and assembling GalN/GalNac-rich moieties of Giardia cyst wall.