This is the first of two papers dealing with the wall structure and growth of Chaetomorpha (Ch. melagonium and Ch. princeps) and of Cladophora (Cl. prolifera and Cl. rupestris). Here the structure is investigated of the side walls and cross-walls of the cells in the filaments, including the apical cells, using polarization microscopy, X-ray diffraction analysis and electron microscopy. All the side walls are finely lamellated, with microfibrils lying almost transversely and almost longitudinally, crossing each other at an angle rather less than a right angle. In Cl. prolifera, in Ch. princeps and, very rarely, in Cladophora rupestris, there is also a third set of microfibrils the direction of which lies in the obtuse angle between the other two. All three sets of microfibrils lie in spirals round the cells and, wherever all three occur, they are deposited by the cytoplasm in the same order: slow, steep, third. The signs of the spirals are constant within a species. The structure of the side walls of all four species-and of the walls of Valonia ventricosa in which the same order of deposition has been found-can be simply related on a grid of three sets of intersecting lines. In newly deposited lamellae the microfibrils irrespective of their orientation show the same considerable angular dispersion about their common direction. Further in the wall, the microfibrils of the steep set are more nearly parallel to each other and of the slow and third set more dispersed. This change is associated with cell extension. The microfibrils of each type of lamella are interwoven within and between lamellae. The microfibrils in the cross-walls, the lamellae of which are continuous with those of the side walls, lie at random, with a tendency near the edge to run parallel and perpendicular to it. In apical cells of Cl. prolifera all three sets of microfibrils pass completely over the growing apex through a random patch at the extreme tip. All three sets are more disoriented, and more widely spaced, in outer than in inner lamellae. The special features of wall deposition consequent upon plasmolysis and subsequent recovery and upon preparation for the release of swarmers are reported upon. Both cases involve the deposition of randomly oriented microfibrils. On plasmolysis cytoplasmic material is left behind on the wall, associated with new microfibrils, and some microfibrils in process of deposition are drawn away from the wall on the surface of the cytoplasm. It is concluded that in these species the cellulose microfibrils are produced by end synthesis and are oriented by a mechanism occupying a thickness of about 500 angstrom which can orient them in any one of three directions simultaneously. Wall stresses are of no significance in initial orientation. The incrusting substances continue to aggregate within the wall so that in formalin-fixed material only the outer lamellae are firmly bound together.