A new gibberellin, gibberellin A20 (GA20), was isolated from immature seeds of morning-glory (Pharbitis nil). Its structure was established as 4aα, 7α-dihydroxy-lβ-methyl-8-methylenegibbane-lα, 10β-dicarboxylie acid-1→4a lactone (I) on the basis of its physico-chemical analysis as well as chemical evidences. GA20 shows marked growth promoting activities on dwarf maize d2 and d5 but weak activities on d1, rice seedling and dwarf pea.