Cathanneine(I), a dihydroindole alkaloid with the molecular formula C₂₄H₃₀N₂O₅, was isolated from the leaves of Catharanthus lanceus (Boj. ex. A. DC.) Pich. Data indicate this new alkaloid contains a cyclic ether moiety. Analyses of its ultraviolet (λmax at 208nm, 255nm, 308nm), infrared (peaks assigned to ester, carbomethoxy, acetate, indoline, and aromatic moieties), nuclear magnetic resonance (compared with vindoline and vindorosine), mass spectral (aspidospermine-type fragmentation, high-resolution measurements supporting fragmentation hypotheses), and other data led to the proposal of structure I for cathanneine. The stereochemical implications are tentative and will be discussed in a later paper.