Portimine, a new polycyclic ether toxin containing a cyclic imine moiety, was isolated from the marine benthic dinoflagellate Vulcanodinium rugosum collected from Northland, New Zealand. The structure of portimine, including the relative configurations, was elucidated by spectroscopic analyses. The cyclic imine moiety consists of an unprecedented five-membered ring with a spiro-link to a cyclohexene ring. This is the only structural similarity to the pinnatoxin group of polycyclic ethers also produced by V. rugosum, which all contain a six-membered cyclic imine ring. The LD50 of portimine to mice by intraperitoneal injection was 1570 μg/kg, indicating a much lower toxicity than many other cyclic imine shellfish toxins. In contrast, portimine was highly toxic to mammalian cells in vitro with an LC50 to P388 cells of 2.7 nM, and activation of caspases indicating apoptotic activity.