1. Cyst walls of the beet cyst-nematode (Heterodera schachtii Schmidt) were obtained by sieving a suspension of crushed cysts; about 15mg of dried cyst walls was obtained from 1000 cysts. 2. The cyst walls contained 68% protein calculated from nitrogen content. Glutamic acid, glycine, proline and hydroxyproline made up about 54% by weight of the amino acids obtained on acid hydrolysis. 3. Minor constituents of the cyst wall were hexosamine (3.3%), lipid (6%), carbohydrate (2%) and phenols (2%). The hexosamine was identified as galactosamine. 4. The cyst walls contained inorganic material (ash 17%), most of which was extractable with EDTA, but not with water. Major inorganic components were calcium and phosphorus (1.7% and 1.5% respectively, by weight). Carbon dioxide (about 1% by weight) was liberated from the cyst walls on acidification. 5. The cyst walls of H. schachtii and the potato cyst-nematode (Heterodera rostochiensis) contained different amounts of the same amino acids. They also differed in their inorganic content and in the nature of the hexosamine present.