Recent investigations of the alkaloids of the Australian Leguminosae have shown that C20-pentacyclic alkaloids of the ormosanins-piptanthine type are not restricted in occurrence to Ormosia and Piptanthus species. Structural assignments in this group are particularly difficult due to the large number of possible stereoisomers—for example, there are sixteen stereoisomers of the fully-reduced C20H35N3 base ormosanine without considering absolute configuration differences. An X-ray crystal structure analysis was carried out on the monohydrobromide of podopetaline (C20H33N3), a new optically-active base that is a major component of the alkaloid mixture from Podopetalum ovatum F. Muell. As the absolute configuration of any Ormosia alkaloid was unknown, this analysis represents the first determination of an absolute configuration for a member of this alkaloid group, providing an absolute chirality reference for the group. Podopetaline has a melting point of 77.5–79°C and [α]D -48° (c 0.95 in methanol); its crystalline monohydrobromide (C20H33N3·HBr) melts at 253–256°C with [α]D -37° (c 1.3 in methanol). Crystals of podopetaline monohydrobromide are orthorhombic with unit cell dimensions a = 6.619 Å, b = 10.907 Å, c = 26.806 Å, space group P212121, and Z = 4. Intensity data for 1558 independent reflections (849 significant) were measured using CuKα radiation on a 4-circle diffractometer. Bromine sites, located from a three-dimensional vector map, were used to derive an approximate electron-density distribution. Subsequent least squares refinements and difference syntheses yielded non-hydrogen atom sites, differentiated nitrogen atoms, and defined the location of the double bond in the molecule. Final least squares refinement resulted in a reliability index R (|ΔF|/|Fo|) of 0.075 for the 849 observed reflections. The absolute chirality of the organic cation of podopetaline hydrobromide, determined by reference to selected Bijvoet pairs, is shown in I, leading to the conventional representation of podopetaline’s absolute structure in II. Podopetaline shares the same relative configuration at C6, C11, and C18 as ormosanine (whose structure is known from an X-ray analysis of jamine—an alkaloid identical to the product of ormosanine and formaldehyde under mild conditions). However, there is an important conformational difference between podopetaline and jamine (and presumably ormosanine): rings A, C, D, and E of podopetaline adopt the chair form, while ring B (containing the C16–C17 double bond) has five carbon atoms essentially coplanar and C8 out-of-plane (sofa conformation). Podopetaline’s chair form for ring C and cis C/D ring junction (nitrogen lone pair at C18 relative to C6–H) contrast with jamine’s boat form for ring C and trans C/D junction. It is likely that jamine (and ormosanine) have the trans C/D conformation due to steric interaction between C16–H and the nitrogen lone pair at N1 in the quinolizidine moiety, which destabilizes the cis form. In podopetaline, the chair-chair conformations for rings C and D and the cis C/D junction are adopted, representing a more stable conformation in the absence of such steric interactions.