Filaments of the red alga Griffithsia flosculosa have been examined by the methods of X-ray analysis, polarization optics, and electron microscopy, including ultra-thin sectioning. The main structural component of the wall consists of cellulose II (mercerized cellulose) organized into microfibrils about 200 angstrom wide and 100 angstrom thick. These are embedded in an amorphous material. The X-ray diagram resembles very closely that of the green alga Ulothrix flacca. Examination of sections in the electronmicroscope shows that the wall is finely lamellated and in addition there are periodic inclusions which appear to be cytoplasmic. The limiting layer of the cytoplasm bordering the vacuole is very distinct; the layer bordering on the wall has a series of peg-like protuberances into wall and cytoplasm. It is tentatively concluded that the site of wall formation lies, not at the cytoplasm-wall interface, but deeper in the cytoplasm. The chromatophores are lamellated much as the chloroplasts are known to be in higher plants. These lamellae consist here, however, of granules whose size (some 220 angstrom diameter) bears no obvious relation to known constituents of the chromatophores.