The main toxins produced by the dinoflagellate Prorocentrum belizeanum, DTX5c and 7-hydroxymethyl-2methylene-octa-4,7 dienyl okadaate, were studied by liquid chromatography-coupled with ion trap mass spectrometry (LC-MS/MS). Their retention times (RTs) and fragmentation patterns were established, in particular those of DTX5c, as there is a lack of data about these water soluble OA sulphated derivatives. As an application of the LC-MS methodology, a sample of the toxic P. belizeanum culture was analysed for these metabolites. Both metabolites were detected in cells as majority compounds, whereas levels of these compounds in the culture media were undetectable.