A survey of the structure of the cell walls of green, brown, and red algae, as seen under light and electron microscopes is in progress. In this report a comparison of the cell wall structure of a brown alga, Dictyota flabellata, and a red alga, Helminthocladia californica, is presented. In Dictyota, typical of the brown algae, the microfibrillar pattern in the apical cells and in the adjacent cells of the thallus tip is reticulate. In mature cells the microfibrils are dominantly parallel in orientation. Pits, fields of closely set pores, are distinctive. The microfibrils in the pit areas are masked by nonfibrillar material. Helminthocladia, with a cell wall characteristic of the red algae, differs from Dictyota in that the microfibrillar pattern is reticulate throughout the thallus. In the pit areas the microfibrils are not masked by amorphous material.