Recently, we have reported the chemical properties of some jellyfish proteinaceous toxins. These were the first chemical characterizations of jellyfish protein toxins to be reported. The isolation of the proteinaceous toxins in their active forms was the key step in the studies. We isolated the toxins from three box jellyfish (Cubozoa) species [Carybdea rastoni (C. rastoni), Carybdea alata (C. alata), and Chiropsalmus quadrigatus (C. quadrigatus)]. These toxins showed lethal toxicity to crustaceans and hemolytic activity. Furthermore, the full-length cDNAs and the deduced amino acid sequences of the toxins were clarified. All of these toxins have a molecular weight of around 45 kDa and their amino acid sequences showed homology with each other. The box jellyfish toxins represent a novel bioactive protein family.