Adriatoxin-B (ATX-B) is the first C(13) terminal truncated YTX analogous metabolite that was isolated from large-scale cultures of the dinoflagellate Protoceratium reticulatum. The structure and conformational behavior of ATX-B was elucidated by a combination of NMR spectroscopy experiments and a conformational analysis. The isolation of this metabolite, which possesses a clear structural relationship with adriatoxin, supports the idea that the real source of these cut down substances is in the microalgae and not in the mollusks, via metabolic degradation like it has been assumed until now.