X-ray crystal structure analysis of maytoline methiodide has defined the constitution and stereochemistry of maytoline, a novel sesquiterpene alkaloid isolated from Maytenus ovatus Loes. Crystals of the methiodide have space group P21 with a = 11.643(3), b = 14.621 (2), c = 10.436(2) Å, β = 97.43(1)°, and Z = 2. The structure was solved by the heavy-atom method and the atomic parameters refined by least-squares techniques to R 0.062 for 1626 independent reflections measured by counter diffractometry. The stereochemistry of the nucleus of the molecule is related to that of dihydro-α-agarofuran but the absolute configuration has not been established.