The major glycosylphosphatidylinositols (GPIs) in African trypanosomes are glycolipid A, the precursor of the variant surface glycoprotein membrane anchor, and glycolipid C, a species identical to glycolipid A except that it contains an acylated inositol. Both glycolipids A and C contain dimyristoyl glycerol and are efficiently labeled with [H-3]myristate in a cell free system. We now report a novel GPI known as lipid X. This GPI is radio-labeled strongly with [H-3]palmitate (and very poorly with [H-3]myristate or [H-3]stearate) in digitonin-permeabilized cells. The structure of lipid X is Man(1)GlcNAc-(2-O-palmitoyl)-D-myo-inositol-1-HPO4-3(lyso-palmitoylglycerol). Metabolically, lipid X exists as an intermediate, and can be detected only under conditions in which its formation is stimulated (e.g. by EDTA) or its breakdown is inhibited (e.g. by Co2+). Lipid X has not been observed previously because these conditions do not support GPI biosynthesis We speculate that lipid X is an intermediate in the catabolism of conventional trypanosome GPIs possibly deriving from breakdown of glycolipid C.