R. J. Lewis, R. S. Norton, I. M. Brereton and C. D. Eccles. Ciguatoxin-2 is a diastereomer of ciguatoxin-3. Toxicon 31, 637-643, 1993.-Ciguatoxin-2, a major ciguatoxin present in the flesh and viscera of ciguateric fishes, has been shown by 1H nuclear magnetic resonance studies (2-dimensional homonuclear Hartman Hahn, nuclear Overhauser effect and decoupling difference experiments) to be a diastereomer of ciguatoxin-3, differing only in stereochemistry at carbon 52 (a quaternary carbon). This difference accounts for the significant changes in the chemical shift of resonances for protons in this region of ciguatoxin-2. Differences between ciguatoxin-1, -2 and -3 involve modifications at only one end of the ciguatoxins (ring M) and modest differences in potency, indicating that this ring contributes to, but is not critical for, high affinity binding of the ciguatoxins to voltage-dependent sodium channels. It is proposed that ciguatoxin-2 originates from a different precursor to the precursor (presumably gambiertoxin-4b) for ciguatoxin-1 and -3, and that both precursors are produced by a common biosynthetic pathway in Gambierdiscus toxicus.