A polar lipid-soluble toxin, spiro-prorocentrimine has been isolated and crystallized in a separation of prorocentrolide from an unknown benthic Prorocentrum species. This compound, C42H69NSO13, has similar macrocyclic structures as prorocentrolide, a toxin from a strain of P. lima, but a smaller macrolide moiety. Like the toxin, prorocentrolide B, from P. maculosum spiro-prorocentrimine contains a sulfonyl substitution on the 6-membered ether ring instead of the 5-membered cyclic ether within the macrolide moiety of prorocentrolide B. The unique feature of the compound is the spiro-linked cyclic imine with the ortho, para-disubstituted 3 °¶-cyclohexene, a characteristic functional group also found in gymnodimine, rather than the hexahydroisoquinoline moiety on both prorocentrolide and prorocentrolide B. Structure of this compound was established by NMR spectroscopy and x-ray crystallography. This is the first compound of this group of marine toxins with a clear stereochemistry.