The structure of a glycosylphosphatidyl inositol-anchored glucoxylan (GPI-glucoxylan) synthesized by the monogenetic trypanosomatid Leptomonas samueli has been determined. The glucoxylan is anchored to the membrane by phytoceramide and an oligosaccharide core, the structure of which is identical to glycoinositolphospholipids (GIPLs) expressed by this protozoan. The glucoxylan chain is linear, containing -->4Glcalpha1-->, -->4Xylbeta1--> and -->3Xylbeta1--> residues. A well defined sequence heterogeneity was analysed in terms of a series of overlapping trisaccharide substructures. A proportion of the chains are capped with a GlcAalpha1-->3Glcalpha1--> sequence. While an average GlcA-capped chain contained 10 Glc and 16 Xyl residues, uncapped chains have a higher molecular mass with an average of 30 Glc and 50 Xyl per chain. We propose a mode of biosynthesis based on the observed structural heterogeneity.