Patterns of cellulose deposition within the cell wall directly affect the size and shape of plant cells, in turn affecting the overall structure of the plant. In the primary wall of Nitella, and in the walls of many other algae and higher plants, a correspondence has been observed between the organization of cellulose microfibrils and cortical microtubules, each lying on opposite sides of the plasma membrane. The present work examines the development of the secondary wall in maturing Nitella internodal cells in an attempt to determine whether a relationship exists between cellulose microfibril organization and the organization of cortical microtubules. Treatments that artificially rearrange or remove microtubules were used, and effects on cellulose organization in the cell wall were examined through the use of transmission electron microscopy in thin sections and by the replica technique. Removal of microtubules in very young cells had a randomizing effect on cellulose organization, but absence of microtubules or their rearrangement did not prevent the formation of secondary walls with characteristic helicoidal patterns. The findings on the timing of helicoidal development in Nitella and the appearance of the innermost surface of helicoidal layers are discussed in relation to models of helicoidal architecture.