Organic Ultrastructure of Diatoms: A Possible TEM Technique Michael A. Gorycki, Ph.D. ABSTRACT The frustules of diatoms, including fossil forms, make them good subjects for scanning electron microscopy. However, the organic structure of living diatoms is difficult to study by transmission electron microscopy (TEM) because they are too thick, and the frustules, being composed of silica, make them poor candidates for ultra-thin sectioning. The frustules also make the organic material, contained within, difficult to examine by phase contrast microscopy. Since diatoms are useful in the study of water quality, and form part of many food chains, it might be helpful if ultra-thin sections of their organic structuring could be made available for examination. A tentative method is described here that allows a monolayer of whole diatoms to be embedded on a plane in Araldite epoxy and then be exposed to hydrofluoric acid so that the siliceous frustules