Seed of Crotalaria axillaris Ait (Leguminosae) yielded 2.5% of an alkaloid mixture consisting of three components. The major alkaloid, for which the name axillarin is suggested, has the composition C18H27NO7, m.p. 205° (decomp.), [α]D + 65.1° (pyridine), hydrochloride m.p. 228°, picrate m.p. 214-216° (decomp.). Structure (I) is proposed for axillarin based on the following evidence: The mass spectrum confirmed the molecular formula and showed major fragments with m/e 136, 120, 119, 93, and 80, a series observed in mass spectra of retronecine macrocyclic diester alkaloids. Hydrogenation (2 moles uptake) followed by alkaline hydrolysis gave retronecanol and a non-crystalline acidic component, confirming the cyclic diester structure. The n.m.r. spectrum (pyridine) indicated three CHMe groups including an isopropyl substituent. The i.r. spectrum (KBr) showed hydroxyl and hydrogen-bonded ester groups (νmax 3490, 3420 and 1730 cm⁻¹). Periodate oxidation demonstrated a 1,2,3-triol system with consumption of two moles of periodate per mole of axillarin and production of acetaldehyde. Reduction with LiAlH₄ gave a neutral product derived from the necic acid, which on treatment with HIO₄ followed by hot 2,4-dinitrophenylhydrazine gave the derivative of 2-isopropylprop-2-enal. The proposed structure for the necic acid component of axillarin (as in (I)) is a new fifth member of C₁₀ necic acids and is probably derived in vivo from valine and isoleucine with loss of the carboxyl group of the latter.