Cellulose microfibrils from the cell wall of the green alga Valonia macrophysa were isolated by enzymatical digestion of the wall matrix and examined in the electron microscope using the negative staining technique. The smallest fibrils obtained after prolonged treatment were found to be flat ribbons with a width about 10—20 nm and a height of 3 to 4 nm. This result is discussed in relation to Frey- Wyssling's concept of an “elementary fibril“ with a cross-section of 3.5 × 3.5 nm. Some alternation of unstained areas along the fibrils was observed. This was interpreted as artificially induced rather than relating to the structure and the arrangement of the cellulose chain molecules.