Two toxic polypeptides BTTX I and BTTX II were isolated from the sea anemone Bolocera tuediae. Toxin isolation was achieved by alcoholic extraction of the homogenized tentacles and filaments, batchwise adsorption onto ion-exchangers (QAE-Sephadex A25, CM-Cellulose, Se-Cellulose, Sp-Sephadex C25), gel filtration on Sephadex G50 and G25 and ion-exchange chromatography on SP-Sephadex C25 and QAE-Sephadex A25. The toxins were tested on the shore crab Carcinus maenas by intramuscular injection. LD 100 for BTTX I is 500 μg kg −1 C. maenas, whereas the BTTX II it is 40 μg kg −1. The molecular weights of both toxins are estimated to be 5000 Dalton.