A breakthrough was made in structural determination of ciguatoxins. High energy collision FAB-MS/MS experiments carried out by selecting Na+ adduct ions as parent ions led to elucidating structures of as many as 20 congeners including new precursor toxins. The structures were further confirmed by charge-remote fragmentation FAB-MS/MS spectrometry after introducing a negative charge group to the molecules. The method required only 10 microgram or less of samples and was applicable to mixtures of congeners. The structural information thus obtained gave a new insight into the diverse biochemical modification of toxins, laid basis to monitoring toxins by LC/MS at pg levels, and provided a new strategy on how to apply immunochemical assays. In the presentation emphasis will be laid on the usefulness of the method to elucidate other marine toxins rather than the details of the analytical methods. Dissemination of our results will be encouraging to the scientists who want structural information on samples which are available only in tiny amounts and in impure forms.