No structural information regarding alkaloids of the family Elaeocarpaceae has previously been reported. We now report that two new alkaloids, elaeocarpine, C₁₇H₁₇NO₃, m.p. 81-82°, [α]ᴅ ca. +0.6° in CHCl₃, and an isomer, isoelaeocarpine, C₁₇H₁₇NO₃, m.p. 51-52°, [α]ᴅ ca. +0.4° in CHCl₃, isolated from the leaves of a New Guinea species Elaeocarpus polydactylus Schl., are the first members of a new class of indolizidine alkaloids. The complete stereochemistry of elaeocarpine (I) has been established by an X-ray crystal structure analysis of elaeocarpine hydrobromide which crystallizes in the triclinic system with two formula units in a unit cell of dimensions a = 12.46 Å, b = 7.38 Å, and c = 8.05 Å, α = 96.1°, β = 92.8° and γ = 94.0°; the N(z) versus z test clearly shows the space group to be Pī, indicating essentially racemic crystals, and the electron-density map (based on bromine atom co-ordinates from the Patterson function) reveals the positions of all C, N, and O atoms. The isomeric alkaloid isoelaeocarpine (II) is closely related to elaeocarpine, as shown by their interconversion in methanolic potassium hydroxide solution at room temperature. Both alkaloids exhibit carbonyl absorption bands (I: ν(CCl₄) 1694 cm⁻¹; II: ν(CCl₄) 1680 cm⁻¹) and similar mass spectra (M⁺, m/e 283) with only minor differences in peak intensities. Their 100 Mc./sec. ¹H NMR spectra show a sharp three-proton singlet assigned to an aryl C-methyl group (δ 2.60 in I, δ 2.63 in II) and similar ABX systems (670-740 c./sec.) for three aromatic protons. NMR analysis (including double irradiation) demonstrates isoelaeocarpine is the C-7 epimer of elaeocarpine: the C-7 proton of elaeocarpine appears as a broad multiplet at δ 4.16 with coupling constants J₇,₈ 11.8 c./sec. and +(J₇,₆β + J₇,₆α) 7.8 c./sec. (consistent with trans-diaxial couplings), while the C-7 proton of isoelaeocarpine is a narrow multiplet at δ 4.64 with small coupling constants (consistent with C-7, C-8 cis-stereochemistry). The C-8 proton signals (quartets at δ 2.61 in I, δ 2.42 in II) confirm identical C-8, C-9 stereochemistry in both alkaloids. Proton assignments in the NMR spectra were confirmed by examining C-8 deuterio-substituted derivatives of I and II in CD₃OD containing a trace of NaOD.