Lycopodium annotinum L. has proven to be a very rich source of alkaloids, over thirty bases having been isolated to date. This paper reports the isolation, characterization, and structure determination of a new alkaloid, annopodine (C₁₇H₂₅O₃N), from this plant. The alkaloid was obtained in a small amount (0.2 g from 250 g crude alkaloid) from the acetone mother liquors remaining after the isolation of α-obscurine, lycodoline, and annotoxine, with a melting point of 211-212° and characterized by its hydroperchlorate (m.p. 210-212°), methiodide (m.p. 240-242°), and hydrobromide (m.p. 230-232°), whose properties do not correspond with those of any known alkaloids. Spectroscopic (infrared, ultraviolet, and n.m.r.) analyses of annopodine, O-acetylannopodine, and anhydroannopodine revealed that annopodine contains a secondary hydroxyl group, a tertiary nitrogen, and an α,β-unsaturated ester, and the molecular formula and functional groups require the molecule to be tetracyclic. X-ray analysis of annopodine hydrobromide (orthorhombic space group P2₁2₁2₁ with four molecules in the unit cell, R-factor of 11.3%) determined its molecular structure, showing annopodine represents a new type of skeleton in Lycopodium alkaloids. The absolute configuration was determined from anomalous dispersion calculations. Dehydration of annopodine gives the unconjugated diene anhydroannopodine. A complete discussion of the chemistry of annopodine and biogenetic implications will be presented in the full paper.