The green-coloured unicellular alga Synechocystis didemni, which has a pigment complement like that of a chlorophyte, is shown by transmission electron microscopy to have a typical prokaryotic construction. The nuclear body is not membrane-bound; there are no mitochondria; the thylakoids are distributed throughout the cell, though often more or less stacked; there are large polyhedral bodies with a crystalline fine structure; and the cell wall is four-layered. In these features S. didemni seems to be a typical cyanophyte.