The major accepters of sialic acid on the surface of metacyclic trypomastigotes, which are the infective forms of Trypanosoma cruzi found in the insect vector, are mucin-like glycoproteins linked to the parasite membrane via glycosylphosphatidylinositol anchors. Here we have compared the lipid and the carbohydrate structure of the glycosylphosphatidylinositol anchors and the O-linked oligosaccharides of the mucins isolated from metacyclic trypomastigotes and noninfective epimastigote forms obtained in culture. The single difference found was in the lipid structure. While the phosphatidylinositol moiety of the epimastigote mucins contains mainly 1-O-hexadecyl-2-O-hexadecanoylphos phatidylinositol, the phosphatidylinositol moiety of the metacyclic trypomastigote mucins contains mostly (~70%) inositol phosphoceramides, consisting of a C18:0 sphinganine long chain base and mainly C24:0 and C16:0 fatty acids. The remaining 30% of the metacyclic phosphatidylinositol moieties are the same alkylacylphosphatidylinositol species found in epimastigotes. In contrast, the glycosylphosphatidylinositol glycan cores of both molecules are very similar, mainly Mana1-2Mana1-2Mana1-6Mana1-4GlcN. The glycans are substituted at the GlcN residue and at the third aMan distal to the GlcN residue by ethanolamine phosphate or 2-aminoethylphosphonate groups. The structures of the desialylated O-linked oligosaccharides of the metacyclic trypomastigote mucin-like molecules, released by ß-elimination with concomitant reduction, are identical to the structures reported for the epimastigote mucins (Previato, J. O., Jones, C., Goncalves, L. P. B., Wait, R., Travassos, L, R., and Mendoca-Previato, L, (1994) Biochem. J. 301, 151-159), In addition, a significant amount of nonsubstituted N-acetylglucosaminitol was released from the mucins of both forms of the parasite, Taken together, these results indicate that when epimastigotes transform into infective metacyclic trypomastigotes, the phosphatidylinositol moiety of the glycosylphosphatidylinositol anchor of the major acceptor of sialic acid is modified, while the glycosylphosphatidylinositol anchor and O-linked sugar chains remain essentially unchanged.