Porantherine, C₁₅H₂₃N, the major alkaloid of Poranthera corymbosa Brogn. (Family Euphorbiaceae) was isolated as colourless crystals, m.p. 36 - 40°C, [α] + 29° (c, 0.35 in CHCl₃), M⁺ at m/e 217, from plants collected near Torrington in northern New South Wales. Porantherine affords a crystalline hydrobromide, C₁₅H₂₃N·HBr, unmelted at 340°C, [α] - 13° (c, 0.35 in ethanol). X-ray diffraction measurements showed these crystals belong to the orthorhombic space group P2₁2₁2₁ with lattice parameters, a = 11.689, b = 15.190 and c = 7.721 Å; 1203 independent intensities were measured on a diffractometer with CuKα radiation. The structure was solved by the heavy atom method and the molecular skeleton I was found. Refinement defined unambiguously the location of the double bond and differentiated the N atom. Dimensions within the molecule are typical within the limits of experimental error. Examination of selected Bijvoet pairs determined the absolute configuration to be that in I. The 3-D shape of the molecule is shown in II. Other spectroscopic data for porantherine are consistent with the structure I. In the 100 MHz n.m.r. spectrum of porantherine, a three-proton singlet at δ 1.13 can be assigned to the C-methyl group protons, a narrow two-proton multiplet at δ 5.69 to both double bond protons, and a broad one-proton singlet at δ 3.70 to C₁₀-H, which is adjacent to both the nitrogen and the double bond. It seems probable that porantherine is derived biosynthetically from condensation of a C₁₀-polyketide chain and one equivalent of ammonia, with appropriate cyclizations and loss of the terminal carboxyl group as in III. This mode of derivation resembles the postulated biosynthesis of (+)-9-aza-1 methylbicyclo[3,3,1] nonan-3-one (IV) an alkaloid found in Euphorbia atoto which belongs to the same family as P. corymbosa.