The absolute configuration of physostigmine has been established by correlating the configuration of its C-3a atom with that of the asymmetric carbon atom in (+)-3-ethyl-3-methoxycarbonyl-3-methylpropionic acid. Comparison of the optical rotatory dispersion spectra of physostigmine, Na-norphysostigmine, geneserine, physovenine and eseramine have shown that all five alkaloids have the same absolute configurations.