AbstractDuring encystment, Giardia trophozoites become encased in a filamentous extracellular matrix of their own making that consists of novel cyst wall proteins (Cwp) 1, 2 and 3, and a novel 2-acetamido-2-deoxy-d-galactan we are naming giardan. Giardan is synthesized from glucose via sugar phosphate intermediates to UDP-GalNAc by inducible, cytosolic enzymes. The UDP-GalNAc is fixed into giardan apparently by an inducible, particle-associated transferase. Regulation of this synthesis appears to centre around pyrophosphorylase, epimerase and cyst wall synthase (Cws) activities. Pyrophosphorylase seems to be involved in making sufficient UDP-N-acetylglucosamine (GlcNAc) to drive the epimerase kinetics toward UDP-GalNAc synthesis, while the Cws removes intracellular UDP-GalNAc, extruding it as giardan and thus preventing an increased intracellular concentration of UDP-GalNAc that could drive the reaction toward GlcNAc synthesis. Cyst wall proteins have been localized to encystment-specific vesicles (ESVs), but whether or not this is true for giardan is unknown. Also unknown is whether or not the cyst wall proteins and giardan are covalently linked. It remains unknown how or whether Giardia degrades giardan during excystation.