The ultrastructure and carbohydrate chemistry of Glaucosphaera vacuolata Korsh. were studied to clarify the uncertain taxonomic position of this alga. Serial thin sections reveal that the double membrane bounded photosynthetic apparatus of Glaucosphaera, described by previous investigators to consist of many chromatophores or cyanelles, is a single, highly lobed chloroplast without a pyrenoid. Thylakoids are single and bear phycobilisomes. The single nucleus is centrally located and surrounded by Golgi bodies. Starch granules are located outside the chloroplast and are not membrane bounded. Chemical analysis of the solubilized starch granules by butanol precipitation, iodine staining, and enzymatic digestion indicates the presence of both amylose and amylopectin. It is concluded that Glaucosphaera is a red alga and should be assigned to the order Porphyridiales.