A chemically resistant cuticle fraction was isolated from 5 phaeophycean, 1 rhodophycean, and 11 chlorophycean marine algae using acid treatment alone, or acid treatment followed by leaching in cupra-ammonium. In Cladophora rupestris and Chaetomorpha melagonium this fraction consists of several alternate microfibrillar and amorphous layers similar in appearance to those seen in innermost carbohydrate-rich regions and amount to about 1/10 or more of the cell wall thickness. In Porphyra umbilicalis and Padina vickersiae it is a single layer less than I μ thick, accounting for 1/50-1/100 of the cell wall in Porphyra, and 1/5-1/10 of the cell wall in Padina. The cuticle fractions of all 4 algae contain surprisingly large amounts of protein (about 70% in Cladophora and 80% in Porphyra). Similarities in the behavior of cuticles obtained from the other 12 species studied suggest that they may have a similar protein-rich composition.