Electron microscope observations onPhycopeltis epiphyton, a subaerial green alga found growing on the leaves of vascular plants and bryophytes, revealed the presence of a densely staining material within the inner and outer zones of the cell walls. This material resists acetolysis, is degraded by chromic acid, is unaffected by ethanolamine and exhibits secondary fluorescence when stained with the fluorochrome Primuline. These characteristics, together with infrared absorption spectra indicate that, on the basis of currently accepted criteria, the densely staining material is a sporopollenin and that it is a major component of the cell wall. Tests for cellulose, chitin, and lignin were negative, and little if any silica is present. It is suggested that negative results in tests for cellulose may be due to a masking effect by the sporopollenin. Comparison of the fine structure of the cell walls ofP. epiphyton, pollen grains, and algal cells (known to contain sporopollenin) supports the suggestion that sporopollenin deposition on “unit membranes” is universal. Morphological similarity among sporopollenin lamellae inP. epiphyton, pollen grains, spores of land plants, and the trilaminar sporopollenin sheath inChlorella, Scenedesmus, andPediastrumindicates that the structures may be analogous. As in pollen grains, sporopollenin may provide protection against desiccation and parasitism. It may also be involved in the adhesion ofPhycopeltisto host plants and in the adhesion between adjacent filaments of the thallus.