Investigations on the mechanism of orientation of the cellulose microfibrils of the green algaOocystis solitaria have been carried out. This organism demonstrates easily observable and highly ordered microfibrils in its wall, which are arranged parallel to one another and regularly alternate at 90‡ from layer to layer of which there are approximately 30. During the entire wall development, and always parallel to one of the microfibril directions, are microtubules lying in the cortical cytoplasm. In the presence of 10−2 M colchicine, microtubules are no longer detected and the typical cell wall pattern is not developed. The possible role of microtubules in the orientation of cellulose microfibrils is briefly discussed.