Two glycosidic toxins, prymnesin-1 (PRM1) and prymnesin-2 (PRM2) have been isolated from cultured cells of the red tide phytoflagellate Prymnesium parVum, and the gross structure of PRM2 C96H136-Cl3NO35 has been reported in a previous contribution. The molecule possesses unique structural features: a C90 unbranched carbon chain except for a single methyl, five contiguous ether rings (6/6/6/7/6), four distinct 1,6-dioxadecalin units, conjugated double and triple bonds, chlorine and nitrogen atoms, and an uncommon L-xylose. Potent ichthyotoxic and hemolytic properties of the two toxins were also demonstrated. However, the stereochemistry of PRM2 and the structure of PRM1 remained unknown. In the present paper the relative stereochemistry of the polycyclic ether moiety of PRM2 and the structure of PRM1 C107H154Cl3NO44 are reported. Structural elucidation was carried out by extensive analysis of NMR data. Difficulties arising from the poor solubility of the toxins in NMR solvents were overcome by preparing N-acetyl, peracetyl, perhydro, and dehydrochlorinated derivatives of the toxins. These derivatives were also effective in improving the precision of signal assignments. 13C NMR measurements were facilitated by a 13C-enriched N-acetate from cultures of the flagellate in the presence of Na13CO3. Comparison of the NMR data between N-acetylprymnesin-1 and N-acetylprymnesin-2 and chiral GC analysis of the glycosidic residues indicated that PRM1 possessed R-Dribofuranosyl, R-L-arabinopyranosyl, and â-D-galactofuranosyl residues at C77, C78, and C82 positions, respectively, on the same aglycone structure as PRM2. In the polycyclic parts (C20-C74) of both toxins, all ring fusions were trans and all rings took a chair conformation. The stereochemistry at C14 and C76-C85 remains unknown.