A new C20-diterpenoid alkaloid, yesoline, and a known alkaloid, 14-dehydrodeloosine were isolated from Aconitum yesoense var. macroyeaoense (Nakai) Tamura. Structure of the new alkaloid was determined by means of spectral and chemical methods. The isolation and structure elucidation of fourteen C20-diterpenoid alkaloids and five C19-diterpenoid alkaloids from Aconitum yesoense var. macroyeaoense (Nakai) Tamura were reported in our previous paper. Our continued investigations on the constituents of this plant now resulted in the isolation of a new diterpenoid alkaloid, yesoline (1), and a known alkaloid, 14-dehydrodelcosine (2). Alkaloid 2 was identified by comparison of the spectral data with those in literature. The ¹H-nmr spectrum of yesoline (1), C30H37NO6, showed signals due to four methyl groups assignable to C-CH3, N-CH3, and two OCH3, signals due to two methines and typical 1,2,4-trisubstituted benzene (See Experimental). The ¹H-nmr, ir, uv, and mass spectral data suggested the presence of veratroyloxy group as an ester. The ¹H-nmr signals at δ 5.23 and 5.41 (each 1H, s, C-CH2), and ¹³C-nmr signals at 144.5 ppm (s, C-16) and 117.1 ppm (t, C-17) suggested the presence of an exo-methylene moiety, and the molecular formula together with the spectral data suggested that the compound was a C20-diterpenoid alkaloid. The ¹³C-nmr spectrum of 1 was similar to that of 15-veratroylpseudokobusine (3), with the exception of presence of a new quartet at 42.8 ppm and a singlet at 203.1 ppm, and the absence of a singlet at ~98 ppm. These data suggested that yesoline was veratroyl ester of N-methyl-N,6-seco-6-dehydropseudokobusine (6). The structure of yesoline (1) was finally confirmed by the correlation with pseudokobusine (4), already isolated from the same plant, as follows. Treatment of pseudokobusine (4) with methyl iodide in methanol gave a methiodide (5), which was stirred with silver oxide in 50% aqueous methanol to give an N-methyl ketone (6). On the other hand, alkaline hydrolysis of 1 afforded N-methyl-N,6-seco-6-dehydropseudokobusine (6). The N-methyl ketone (6) derived from pseudokobusine and the product derived by the hydrolysis of yesoline showed identical ir and nmr spectra, and the tlc behaviors.