The structure of a new ciguatoxin congener. CTX3C (I), isolated from cultures of a dinoflagellate Gumhierdisc~r.~ fmicur was elucidated mainly on the basis of NMR data. I closely resembles gambiettoxiw4b (2). a known cigualoxin precursor, but lacks the butadiine side chain in 2 and has oxocene ring E instead of oxopene in 2. Ciguatoxin (CTX) and ils congeners play the most important role in ciguatera fish poisoning prevalent in circumtropical regions. The structural similarity shared by CTX isolated from moray eels and gambiertoxin-4b (GT4b) isolated from wild specimens of the dinoflagellate Gurnhierdiscu.~ toxic*us suggested the. organism to be the biogenetic origin of CTX.1 Yet, attempts to isolate GT4b or related toxins from dinoflagellate cullures have been unsuccessful in many laboratories, thus casting doubt on the true origin of GT4b.2 In view of the known intraspecfic variation of secondary metabolites in microorganisms, we grew three additional clones of G.toxicur for screening and found one. clone coded RGI-I to produce toxins resembling CI'X. One of them was indistinguishable by chromatographic and MS data from CTX3C, an unelucidated minor toxin of moray eels. In this letter, we report the structural elucidation of CTX3C (1) of G. tr>xic*u.v. The RGI-I clone of G. toxic-us was isolated at Rangima Atoll, the Tuamotu Archipelago, French Polynesia, and cultured in a sea water medium.3 Methanol extracts of the cells were partitioned between aqueous 60% MeOH and CH2Cl2 to obtain CTX3C in the organic phase. Subsequently, 1 was purified by chromatography over Florisil (Me2COFleOH 9: I), Toyopearl HW-40 (MeOH) and Asahipak ODP-50 (MeCN/H20 75:25). From I, IO0 L of the culture. 0.7 mg of CTX3C was obtained: mouse lethality, I .3 l,tg/kg (i.p); HR-FABMS , MH+ m/z 1023.5660 (calcd. for C57Hg2016 + H, m/z 1023.5680). Structural elucidation of CTX3C was achieved mainly by measuring homo 2D NMR: e.g.. COSY and TOCSY and NOESY (JEOL GSX-400, 4OOMHz, in CgDgN solution). The spectra of 1 closely resembled those of GT4b (2). In particular. connectivities. chemical shifts, signal shapes. and NOES of protons from C26 through C57 in 1 agreed well with those of the corresponding signals in 2 (H29 through H6Q.l Thus they shared the same structure at that part, and the structural alteration resided somewhere between Cl and C2S in 1 (Fig. I). Absence in 1 of the butadiene side chain was evident from the lack of corresponding signals and the appearance of new signals due to Ha,p-I (6 4.05. 4.35 ppm) in the lH NMR spectrum of 1. The lH NMR signals (at 23ยฐC) around ring F were extremely broadened due to the conformation change, as were the case with 2. The 2D spectra were, therefore, measured at -20ยฐC and led to elucidating all proton connectivities from HI to 1975