A new species of morning glory,Ipomoea expansa, is described from Guerrero and Oaxaca, Mecico. Morphologically it is very similar toI. purga (Wender.) Hayne and other species of the subsectionEmeticae House, but it is easily distinguished by its lavender-blue corolla and dimorphic leaf morphology. The systematic implications of its discovery for the circumscription of the subsection to which it belongs is discussed.