The new alkaloid Swazine (I), C₁₉H₂₃NO₆, m.p. 165°, (α)ᴅ²⁵ -103.5° (EtOH), λmax (H₂O) 195nm (ε 11,000), was obtained from Senecio swazensis Compton using the standard extraction procedure. The nitrogenous base was readily shown to be retronecine. Chemical and physical studies together with a crystal structure determination of a spiro-dilactone obtained by hydrolysis in 3N sulphuric acid yielded the structure of the diacid moiety and hence an acceptable structure for the alkaloid itself, but subsequent mass spectroscopic data cast doubt on the proposed mode of attachment of the diacid chain to the retronecine nucleus, prompting a complete structure determination. Crystals of the methiodide C₁₉H₂₆NO₆I (supplied by Dr. C.G. Gordon-Gray) were monoclinic, space group P2₁, with a = 8.975 Å, b = 13.125 Å, c = 9.535 Å (± 0.005 Å), β = 115.70°, Z = 2. Intensity data were measured on a diffractometer using CuKα radiation. The structure was solved by the heavy atom method and refined isotropically by block diagonal least squares to R = 0.13 for 1400 observed data (all calculations done with a local set of programs; refinement is continuing). The points of attachment between the diacid chain and the retronecine nucleus are reversed relative to those previously suggested. The presence of an epoxide group in the chain is confirmed, and its chemical inertness is due to oxygen atom O(2) being shielded by atoms O(6), O(3) and C(6) (O(2)....O(6) 3.02 Å, O(2)....O(3) 3.11 Å, O(2)....C(6) 2.77 Å). Bond lengths and angles observed in the retronecine unit are quite usual and very similar to those in jacobine, with the saturated 5-membered ring being exo buckled. The configuration of the 12-atom diester ring is similar to that found in both jacobine and clivorine, where the two ester C=O groups are close to co-linear and point in opposite directions on either side of the mean plane of the 12-membered ring; this conformation is determined by non-bonded contacts across the ring (C(11)....O(5) 3.55 Å, C(10)....O(5) 3.65 Å, O(6)....O(5) 3.19 Å). Atoms C(5), C(10), O(4), O(6) and C(11) are coplanar, with O(1) only 0.2 Å out of the plane and O(1)....O(4) = 2.69 Å suggesting the presence of an intramolecular hydrogen bond. The groups C=CH₂ and C=O are approximately cis-oriented (dihedral angle of 54° between them), implying little or no conjugation and explaining why the UV absorption is at a shorter wavelength than normally expected for the CH₂=C-C=O grouping (this anomaly led to early confusion in reconciling NMR and UV spectra interpretations).