ESERINE (physostigmine) was first isolated from the Calabar bean in the latter half of the last century1, but it was not until much later that its activity on the heart was demonstrated to be due to inhibition of acetylcholinesterase2,3. It is one of the few compounds that are equally active as inhibitors of both true and pseudo-cholinesterase and has been used by many workers as a reference standard in the evaluation of new cholinesterase inhibitors. The chemical structure of this alkaloid was elucidated by Stedman4 and the absolute configuration has relatively recently been confirmed by use of the nuclear Overhauser effect (NOE) in NMR spectroscopy5. As part of our study of the geometry of molecules affecting cholinergic nervous transmission, we have determined the crystal structure.