The major water-soluble arsenic compound was isolated from the muscle of blue pointer Isurus oxyrhincus and whitetip shark Carcarhinus longimanus. The isolation was achieved by a combination of a variety of chromatography. This arsenic compound was adsorbed to Dowex 50W-×8 (H⁺ form), not adsorbed to AG 1-×8 (OH^- form) and active carbon, and weakly adsorbed to Dowex 50-W×8 (pyridinium form). It was identified as arsenobetaine by comparison of thin layer chromatographic behavior, infrared and fast atom bombardment (FAB) mass spectra with those of a synthetic arsenobetaine.