Crotalaria semperflorens Vent. is a shrub growing at altitudes above 6000 ft in the Nilgiri and Pulney Hills, Western Ghats, India. Its seed contains at least three alkaloids, and we report here the isolation and structure of one of them, crosemperine, C₁₉H₃₁NO₆, m.p. 117-118℃. A C₁₉ formula is uncommon among pyrrolizidine alkaloids, being found only in the otonecine esters from Senecio and Crotalaria species (e.g., retusamine). This empirical formula was supported by a parent ion at m/z 367 in the mass spectrum of crosemperine, which also showed fragment ions characteristic of otonecine esters. The nuclear magnetic resonance spectrum of crosemperine clearly indicates it is a macrocyclic diester of an otonecine-type amino alcohol: multiplets at δ 6.0, 5.07, 4.92, and 4.61 correspond to the C2 olefinic proton, C7 proton, and non-equivalent side-chain C9 protons, respectively; the relatively small shift difference (0.31 p.p.m.) of the H9 protons is consistent with an 11-membered rather than a 12-membered ring; the lack of signals around δ 4.0 p.p.m. is consistent with retusamine, otosenine, and senkirkine but not heliotridine or retronecine; a narrow 2-proton multiplet centred near δ 3.2 (with overlapping OH signals) and a signal at δ 2.17 assigned to N-methyl are also characteristic of this group. The higher-field methyl group signals were initially difficult to assign, but a 100 Mc/s spectrum showed the δ 1.25 line was due to an impurity, and the remaining signals were consistent with (CH₃)₂CH (δ 0.96, 0.93), CH₃-CH (δ 1.07), and CH₃-C-OH (δ 1.40), leading to the deduction that crosemperine was probably (I), an ester of otonecine or a diastereoisomer with incanic acid (II) or a diastereoisomer. Alkaline hydrolysis of crosemperine gave a mixture of acids and an intractable basic product. Acid hydrolysis gave an acid, C₁₁H₂₀O₄, with physical properties, infrared, and n.m.r. spectra consistent with incanic acid (II). The basic product from acid hydrolysis, obtained as a hydrochloride, had an n.m.r. spectrum consistent with otonecine. Hydrogenolysis of crosemperine yielded incanic acid and dihydrodeoxyotonecine (III), whose hydrochloride was identified by direct comparison with authentic material. This confirms that crosemperine has structure (I), a diester of otonecine with incanic acid. Incanic acid was previously found only in the alkaloid incanine from Trichodesma incanum (Bgl.) D.C., in which it is esterified with retronecine.