A new natural product has been obtained from the roots of Carrizo citrange rootstocks (Citrus sinensis x Citrus trifoliata) infected by the nematode Tylenchulus semipenetrans, and its structure identified as the diterpene carnosic acid-11-methylether (1) on the basis of spectroscopic data and chemical means. The known compounds beta-sitosterol (sterol), lupeol (triterpene), 5-hydroxy-3,7,3',4'-tretramethoxy-flavone and 5-hydroxy-6,7,3',4'-tetramethoxy-flavone (flavones), citracridone-I (alkaloid) and xanthyletin, nordentatin and poncitrin (chromenecoumarins), were isolated from the same source.