Thin sections of the red alga, Lomentaria baileyana, a tubular member of the Rhodymeniales, were examined after permanganate fixation and Araldite embedding. Many of the cellular structures in Lomentaria were found to be similar to analogous structures in animals and higher plants. However, in the walls between cells are modified areas generally known as pits which are unique to the higher orders of red algae (Florideae). In this study the pits were found to consist of a plug-like structure surrounded by an uninterrupted membrane apparently continuous with the plasma membrane. Examination of the chromatophore revealed a characteristic limiting membrane, a relatively sparse distribution of plates, no grana, and a single disc apparently oriented parallel to the limiting membrane. In addition to their origin from non-lamellate proplastids, chromatophores were found capable of division by simple constriction. Floridean starch grains were observed outside the chromatophore and the possibility of an association of the first formed grains with portions of the endoplasmic reticulum is considered. Gland ceils seem to have a high proportion of Golgi components (dictyosomes), and are believed to have some kind of secretory function. Many of the Golgi vesicles seem to open on the wall and presumably discharge their contents.In view of the paucity of available knowledge on the fine structure of the red algae, a detailed investigation of certain features of this group has seemed warranted. Pits, gland cells, mitochondria, and the origin of chromatophores have received little attention in previous electron microscope studies, and the present work was undertaken to further clarify the structure of some of these organelles.Brody and Vatter (1959) examined the uhrastructure of the unicellular red alga Porphyridium cruentum to determine the relative positions of the phycobilins and the chlorophylls in the cell as well as the influence of different light intensities on cell structure. Myers et al. (1959) published photographs of the pits in Laurencia and Rhodymenia, and cell wall structure has been examined to some extent by Cronshaw et al. (1958) and Myers et al. (1956), while Mitrakos (1960 observed the fine structure of pyrenoids and chromatophores in five genera. Other than these investigations there appear to be no published studies on the ultrastructure of the red algae.The red algae are characterized by the presence of accessory phycobilin pigments, a unique reproduction system, and the absence of motile spores. Other features--the ones investigated in the present study--are the formation of the carbohydrate food reserve (Floridean starch) outside the chromatophore, the presence of pits between cells of the higher orders, and the occurrence of "gland" cells in certain members of the group. The chlorophyllcontaining body in these plants has been variously referred to as the chloroplast, rhodoplast, or 553 on