Arsenic occurs in marine waters, typically at concentrations of 1 - 2 g As/L, present primarily as the inorganic species arsenate. Marine animals are able to accumulate arsenic ? they contain high levels (typically 2,000 - 20,000 g As/kg wet mass), and most of this arsenic is present as the simple organometallic compound arsenobetaine (Me3As+CH2COO-). Although arsenobetaine was first reported in marine animals in the late 1970s, the origin of this compound remains unknown. Arsenobetaine has never been reported in seawater; direct methods of analysis are restricted by the complex seawater matrix, and indirect methods suggest that if arsenobetaine were present, the levels would likely be < 0.1 g As/L. The aim of this work was to develop a method to detect and possibly quantify trace amounts of arsenobetaine in marine waters. Furthermore, the method was applied to samples of seawater collected at various depths from two sites (Sargasso Sea, Atlantic Ocean), with the aim of obtaining preliminary data on the depth profile of arsenobetaine in the ocean. The method involved removing the complex salt matrix by preparative cation-exchange (Dowex 50 H+), while simultaneously concentrating arsenobetaine, and measuring arsenobetaine and other cationic arsenic species by HPLC/mass spectrometry. With this method it was possible to quantify arsenobetaine and seven other cationic organoarsenicals, six of which had never been found in marine waters before. Depth profiles (6 - 200 m) for two sampling sites revealed arsenobetaine concentrations between 0.5 and 14 ng As/L with maxima occurring in the euphotic zone (down to ca 30 m) indicating positive correlation to zones of maximum primary production. The data for the other organoarsenicals found in this study also suggest correlation to primary production with a clear decrease of all arsenic species at depths below 60 meters.