Although there is indirect evidence to suggest that glycogen is present in G. lamblia, to date it has not been purified and identified from this organism. In this study, a high molecular weight carbohydrate was purified and characterized and its physiological role as an energetic reserve was established. The monosaccharide constituents of the carbohydrate reserve were identified as glucose by two independent methods: thin layer chromatography and an enzymatic assay. The degree of branching of the molecule was evaluated by comparing its absorbance spectrum in the presence of lugol with spectra of standard solutions of glycogen and starch under the same conditions. The results strongly suggest that glycogen is present in G. lamblia and acts as an energy reserve in trophozoites of this organism.