In the course of a continuing search for tumor inhibitors of plant origin, alcoholic extracts of the fruits of Croton macrostachys Hochst. ex A. Rich. (Euphorbiaceae) showed significant inhibitory activity in Lewis lung carcinoma in mice (LL). We report herein the isolation and structural elucidation of crotepoxide, a novel tumor-inhibitory cyclohexane diepoxide derivative from C. macrostachys.