1. Erabutoxin b, a neurotoxic protein produced by a sea snake, Laticauda semifasciata, was iodinated at pH 9.6. A toxic derivative containing two iodine atoms per mole, was isolated from the reaction products by CM-cellulose column chromatography. 2. The diiodo-erabutoxin b is chromatographically homogeneous, and crystallizable. The amino acid analysis of the preparation showed the loss of one of the two histidyl residues of the original toxin. Diiodohistidine was detected in the pronase digest of the toxin derivative. Erabutoxin a, which differs from erabutoxin b only in that the former contains asparagine instead of histidine of erabutoxin b at the 26th position of its 62 amino acid residues, did not give a similar diiodo-derivative under the same conditions. It was, therefore, concluded that the diiodo-erabutoxin b has diiodohistidine at the 26th position. 3. The diiodo-erabutoxin b had the same toxicity as the native erabutoxin b. In immunodiffusion experiments, the diiodo-erabutoxin b formed a precipitation line with antierabutoxin b serum of rabbit.