The composition of purified wall fragments from the spherule wall of Physarum polycephalum has been studied. 2. Spherule wall fragments consist of two major components. More than one-half of the wall material is glycoproteins. The remaining wall material after fractionation has a high carbohydrate to protein ratio and is suggested to be composed of proteoglycan(s). 3. The predominant monosaccharide constituent of the proteoglycan is galactosamine. The protein component is unique. It is composed almost entirely of phenylalanine (87--92 mol%). 4. Approx. 50% of these proteoglycans are resistant to alkali extraction and in negative stained preparation for the electron microscope consist of short spindle-shaped fibrils which measure 100 nm in length and 8 nm in diameter. These fibrils show a weak but distinct X-ray diffraction pattern.
