<jats:p> The isolation and determination of the structure of luciduline, C<jats:sub>13</jats:sub>H<jats:sub>21</jats:sub>ON, an alkaloid present among the weak bases of Lycopodium lucidulum Michx, is described. Chemical and physical evidence which suggests structure 9 for luciduline is presented. An X-ray diffraction study on O-p -bromobenzoyl-dihydroluciduline confirmed this hypothesis and defined the stereochemistry of the methyl group at C-8. The complete structure of luciduline, including absolute configuration, is thus represented by 1. A possible mode of biosynthesis of luciduline, which is a new type of Lycopodium alkaloid, is discussed. The mass spectra of luciduline and some of its derivatives are interpreted in terms of structure 1.