Physiological and ultrastructural studies of unialgal cultures demonstrated that silicon is a required nutrient and a component of the cell walls of Cladophora glomerata. Addition of silicon promoted growth of the alga, and the addition of germanium, an analogue of silicon, inhibited growth of C. glomerata in culture. An electron-dense outer layer was a conspicuous part of the cell walls of filaments taken from silicon rich medium.