Hemitheion (1), a new sulfur-containing vobasane-type indole alkaloid, was isolated, together with three known compounds, vobasine (2), gelsedine (3), and gelsemicine (4), from the alkaloid extract of the stems ofMostuea brunonisDidr. (Gelsemiaceae). Compound1could be straightforwardly isolated. Its structure was elucidated by a combination of spectroscopic methods. Besides corresponding to a formerly postulated biosynthetic intermediate toward theionbrunonines, hemitheion (1) stands among the few monomeric vobasanes lacking an oxygen at C-3. Hemitheion (1) showed moderate antiplasmodial activity in the micromolar range against the strain FcB1 ofPlasmodium falciparumand no cytotoxic activity against the MRC-5 cell line at 20 μM. © 2021 American Chemical Society and American Society of Pharmacognosy