The sea anemone (Actinia equina) was newly established to contain a polypeptide toxin (named Ae I) having lethal activity to crabs, besides the well-known cytolytic toxins (equinatoxins) of proteinic nature. Ae I, with a minimum lethal dose against crabs of 25 micrograms/kg, was easily isolated by gel filtration on Sephadex G-50 and reverse-phase HPLC on Nucleosil 300-7C18. Its amino acid composition is characterized by the abundance of Gly, the absence of Ala and the presence of Met. The complete amino acid sequence of Ae I was determined. Ae I has high sequence homology with type 1 sea anemone neurotoxins. Interestingly, the polypeptide chain of Ae I comprises 54 amino acid residues, being 5-8 residues longer than the known type 1 toxins having 46-49 residues.