The wall structures of Cladophora, Chaetomorpha and Spongomorpha have been examined by X-ray diffraction analysis and by electron microscopy, and earlier results have been confirmed and extended. The walls of the two former species contain microfibrils of cellulose I lying in two sets crossing each other approximately at right angles. The walls of the latter species contain a substance resembling though not identical with cellulose II, the microfibrils of which are arranged at random. Examination of developing spores show that in all three species the first wall laid down on the surface of the spore consists of randomly arranged microfibrils which appear to be formed of neither cellulose I nor cellulose II. In Cladophora and Chaetomorpha this is followed by the development, presumably in the cytoplasm, of coarse bands oriented transversely to an axis passing through the point of attachment. Within a few hours, transversely oriented microfibrils appear in these coarse bands and longitudinally oriented coarse bands begin to appear below them. At this stage, namely, at one to two days after the settling of the spore, the mature wall structure is already foreshadowed with two 'poles', one at the point of attachment from which the rhizoid will grow and one at the antipodes. Longitudinally oriented microfibrils subsequently appear in the longitudinal coarse bands giving the typical crossed microfibril structure. The indications are that these oriented microfibrils are constituted from the first of cellulose I. In Spongomorpha neither the coarse bands nor the oriented microfibrils develop and the microfibrils never come to consist of cellulose I. With all three species the wall of the sprouting rhizoid consists solely of microfibrils arranged at random. The implications of these findings with respect to the growth of these algae are discussed.