The major toxin involved in the diarrhetic shellfish poisoning was isolated from the hepatopancreas of the mussel Mytilus edulis and was named dinophysistoxin-1. It was obtained as a colorless solid: mp. 134°C, [d]²⁰D 28°, LD₉₉ 160μg/kg (ip to mouse). Comparison of the toxin by various spectrometries with okadaic acid isolated from the marine dinoflagellate Prorocentrum lima established that dinophysistoxin-1 is 35S-methyl okadaic acid C₄₅H₇₀O₁₃. Gas chromatographic analysis evidenced the presence of dinophysistoxin-l in the dinoflagellate Dinophysis fortii, confirming that this organism is the progenitor of the toxin.