Ordered microfibrils are formed on the membrane of the cytoplasmic tail of the alga Poteriochromonas after attachment to a substrate. The ultrastructure of native and extracted stalk fibrils was studied with electron microscope methods. In addition, the structural polysaccharide was characterized by hydrolyses, separation of the monomers by thin-layer chromatography, gas-liquid chromatography and amino acid analysis, and by X-ray diffraction. The alkali-resistant fibrils yielded mostly glucosamine upon extensive hydrolysis, and showed X-ray diffraction patterns similar to those of fugal chitin. It is concluded that the resistant core of the fibrils is chitinous.