Evidence for the structure of the antitumor alkaloid leurosine is presented, together with the explanations for the mass spectral fragmentation patterns observed. Nuclear magnetic resonance spectroscopic data support the proposal that leurosine contains an epoxide moiety, and is closely related to the other Catharanthus antitumor alkaloids vincaleukoblastine and leurosidine. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company