The cyst wall ofGiardia lambliais essential for survival of the parasite outside the host.N-acetyl-d-glucosamine (GalNAc) has been reported as a major terminal sugar of cyst wall glycoproteins andN-acetyl-d-galactosamine (GalNAc) as the major sugar of the fibrous insoluble cyst wall fraction. Therefore, we measured UDP-glycosyltransferase activities as the incorporation of [ 3 H]UDP-sugar into an alcohol-insoluble product. We found that during encystation only UDP-GlcNAc and UDP-GalNAc transferase (UDP-GT) activities increased approximately three- to fivefold compared to nonencysting trophozoites. These activities were distributed approximately equally in the pellet and soluble fractions. The apparentK m and V max of UDP-GT in these fractions were similar. The activities from both fractions were dependent on Mn 2+ ; however, the pellet enzymes were also partially activated by other metal ions. Both pUDP-GT and sUDP-GT were inhibited by uridine, UDP, and UDP sugars, but not by GlcNAc or GalNAc. Isolation and analysis of the reaction products suggest that pUDP-GT incorporate GlcNAc and GalNAc into glycoproteins, since the products were proteinase sensitive. In contrast, sUDP-GT products were resistant to proteinase treatment. Hydrolysis of the product of UDP-GlcNAc-T incorporation by trifluoroacetic acid released only glucosamine, while both glucosamine and galactosamine were released from UDP-GalNAc-T products, supporting the presence of an epimerase inGiardiawhich can convert GalNAc to GlcNAc during incorporation. This study suggests that at least two UDP-GT activities are induced during encystation, which are responsible for the transfer of GlcNAc and GalNAc from UDP-GlcNAc or UDP-GalNAc into both proteinase-sensitive and proteinase-resistant components of theGiardiacyst wall.