1. Cyst walls of the potato cyst-nematode (Heterodera rostochiensis Woll.) were isolated by sieving a suspension of crushed cysts. About 12mg. of dried cyst walls was obtained from 1000 cysts. 2. The cyst walls contained mainly protein (72%, calculated from nitrogen content). On acid hydrolysis about 77% of the cyst wall went into solution. Of 19 amino acids present, proline, glycine, and alanine were the most abundant, and made up about 50% by weight of the total amino acids. The amino acid composition suggested that collagen-like proteins predominated in the cyst wall and larval cuticle. 3. A small amount of glucosamine (1.5%) was present in the hydrolysates, but chitin was not detected in the cyst walls. 4. Other components of the cyst walls were lipid (2%), carbohydrate (0.5%) and a small amount of inorganic matter (ash, 5%). Polyphenols (2% by wt. of the cyst walls) occurred in the acid hydrolysates. The dark pigments of the cyst wall were not indole-containing melanins.