The chemical structure of a novel lactone isolated from the discolored heartwood of living western red cedar (WRC) (ThujaplicataDonn.) trees has been determined as 3,3,4,7,7,8-hexamethyl-2,6-dioxa-1,5-anthracene-dione, and given the trivial name thujin (1). The isolation, purification, and determination of the structure were carried out by a combination of chemical, chromatographic, and spectroscopic methods.