Thin layer chromatograms for phospholipids obtained from 11 human Giardia lamblia isolates and their culture media have shown that phosphatidylcholine and sphingomyelin are the predominant phospholipid classes in all samples. A decrease in the relative percentage of the different classes, especially of phosphatidylcholine, was noticed in the medium after Giardia growth. Fatty acid analysis of the parasite phosphatidylcholine demonstrated that while oleate and palmitate were the major fatty acids in most isolates, arachidonate predominated in two of those studied. Some isolates contained small amounts of myristate, which was not present in the phosphatidylcholine of the culture medium. Moreover, stearate and linoleate predominated in phosphatidylcholine obtained from both media types. The saturated/unsaturated fatty acid ratio also varied for the different isolates. These results appear to suggest heterogeneity in the metabolic activity and utilization of lipid molecules between Giardia isolates.