Three distinct populations of Ambrosia psilostachya and four specimens of A. acanthicarpa, two of them being seedling and mature plants from a single location, have been examined and their sesquiterpene lactones isolated. Different populations vary markedly in lactone content, and the seedling and mature forms showed a wide qualitative difference. A new lactone, cumanin-3-acetate, and cumanin diacetate, not before found in nature, were isolated from A. psilostachya. Seven other lactones, all known in Ambrosia species, were also isolated. © 1970.