The structure of the diatom, Thalassiosira fluviatilis, was investigated by a study with the electron microscope of thin sections of cells fixed by OsO4, KMnO4, and glutaraldehyde. The weakly siliceous cell wall is composed of a thin sheet perforated by holes about 250 Å in diameter and arranged in a roughly orthogonal manner. At the ends of the barrel-shaped cell are marginal and central pores composed of an outer cylinder (0.25 μ in diameter and 1 μ long) arranged coaxially with an inner, smaller, shorter cylinder. The thin, perforated sheet (valve) which connects these pores is reinforced from place to place by radial or transverse ribs.Mitochondria, nuclei, chloroplasts, and pyrenoids have the same general structure and appearance as those in other diatoms. Small vacuoles are numerous but it is not known whether they are filled with gas or solution. The cells often show aggregates of an amorphous material which does not stain like lipid or protein but which may be the residue of a gelatinous, reserve carbohydrate (chrysolaminarin).