Root exudates from the allelopathic plant, black oat (Avena strigosa Schreb.), were found to contain at least six different germination stimulants for root parasitic plants, but no known strigolactones (SLs). One of these germination stimulants was purified and named avenaol. Its HR-ESI-TOFMS analysis indicated that the molecular formula of avenaol is C20H24O7, and thus it contains an additional carbon compared with known C19-SLs. Its structure was determined as 5-((E)-(5-(3-hydroxy-1,5,5-trimethyl-2-oxobicyclo[4.1.0]heptan-7-yl)-2-oxodihydrofuran-3(2H)-ylidene)methoxy)-3-methylfuran-2(5H)-one, by 1D and 2D NMR spectroscopy, and ESI- and EI-MS spectrometry. Although avenaol contains the C-D moiety, the common structural feature for all known SLs, it lacks the B ring and has an additional carbon atom between the A and C rings. Avenaol is a potent germination stimulant of Phelipanche ramosa seeds, but only a weak stimulant for seeds of Striga hermonthica and Orobanche minor.