The molecular structure of malagashanine (C23H30N2O4), a new alkaloid occurring in Madagascan Strychnos species and showing chloroquine-potentiating action, has been determined by single crystal X-ray diffraction. The crystals, in the form of a 1:1 acetone solvate, are monoclinic, space group P2(1), with a = 10.328(2), b = 10.554(6), c = 11.490(5)Angstrom, beta = 96.67(2)degrees, Z = 2. Refinement based on 3650 reflections measured at 294K yielded R = 0.045. The X-ray analysis shows that the ring system in malagashanine differs from that in alkaloids of the strychnobrasiline type in containing only five- and six-membered rings. Thermal analysis of the solvate indicates that the acetone molecules are firmly held in the crystal and the X-ray analysis reveals that they are trapped in crystal voids. Only van der Waals forces and C-H ... O hydrogen bonds stabilize the crystal structure.