From the methanolic extract of the trunk-bark of Hernandia ovigera L. (Hernandiaceae) collected in Ogasawara island, we isolated a non-phenolic oxoaporphine alkaloid, hernandonine, in addition to the known aporphine alkaloids hernovine (ovigerine, I, R=H), hernangerine (nandigerine, II), and their N-methyl derivatives. Hernandonine, sparingly soluble in common organic solvents, crystallized from chloroform solution in orange-yellow needles, m. p. > 280°. The high resolution mass spectrometry confirmed the molecular formula as C₁₈H₉O₅N. Hernandonine showed a conjugated carbonyl group in the i. r. spectrum in nujol at 1650 cm⁻¹, and its u. v. spectrum in ethanol showed the following absorptions: λmax nm (logε): 222(4.55), 265(4.37), 364(4.03), and 426(3.99). Its n. m. r. spectrum in deuteriochloroform, in which all the nine protons were accounted for, revealed two methylenedioxy groups at δ 6.10 and 6.20. In the aromatic proton region, there was a one-proton singlet at δ7.07, and two AB-type quartets centered at δ6.98, 8.21(J=8.5 Hz), and at δ7.05, 8.80 (J=5.0 Hz). The mass spectrum exhibited the molecular ion peak at m/e 319 as the base peak. Based on these data, we suggested the structure III for hernandonine. Chemical proof for the structure was obtained by the synthesis of III from N-methylhernovine (I, R=CH₃) according to the method previously reported by Tomita et al. Oxidation of N-methylhernovine (I, R=CH₃) with chromium trioxide in pyridine solution afforded an aromatic oxoaporphine, 1,2,10,11-bismethylenedioxydibenzo[de,g]quinolin-7-one (III), in 8% yield, which had m. p. >280°, u. v. spectrum (λmax nm (logε): 222(4.53), 265(4.36), 364(4.00), and 426(4.06)) and i. r. spectrum in nujol identical with those of hernandonine. Consequently, the structure of hernandonine is unambiguously assigned to the formula III.