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, C₁₉H₂₉NO₇, m.p. 117-118°. A C₁₉ formula is uncommon among pyrrolizidine alkaloids, being found only in otonecine esters. The empirical formula was supported by a parent ion m/e 367 in the mass spectrum, which also showed fragment ions characteristic of otonecine esters. The nuclear magnetic resonance spectrum indicated a macrocyclic diester of an otonecine-type amino alcohol: multiplets at δ 6.0, 5.07, 4.92, and 4.61 for the C2 olefinic proton, C7 proton, and non-equivalent C9 protons (a small H9 shift difference of 0.31 p.p.m. suggesting an 11-membered ring), a narrow 2-proton multiplet centred near δ 3.2 (overlapping OH signals), a δ 2.17 signal for N-methyl, and methyl signals consistent with (CH₃)₂CH, CH₃-CH, and CH₃-C-OH. These data suggested crosemperine was probably a diester of otonecine (or a diastereoisomer) with incanic acid (or a diastereoisomer). Acid hydrolysis of crosemperine gave incanic acid (C₁₁H₂₀O₄) with physical and spectral properties matching reported values. Hydrogenolysis yielded incanic acid and dihydrodeoxyotonecine, whose hydrochloride was identified by direct comparison with authentic material. These results confirm that crosemperine has the structure of a diester of otonecine with incanic acid.