The stalked tunicae of the chrysomonadine Ochromonas malhamensis consist of mirofibrils which in contrast to often described plant cellulose fibrils do not have the appearance of one-layer ribbons, but are composed of 10-20 Å thick elementary fibrils, that are very loosely fasciated. The reason for this uncommon structure is thought to be found in the function of the tunicae. Since the elementary fibrils of Ochromonas extend unilaterally, electron microscope studies could be made on the growing ends of the fibrils, several types of which could be discerned. The importance of these structures to the different theories of the growth and the molecular structure of native cellulose is discussed.