Arsenic occurs in marine waters, typically at concentrations of 1-2 ?g As kg-1, primarily as the inorganic species arsenate. Marine animals, however, contain extremely high levels of arsenic (typically 2000-20?000 ?g As kg-1 wet mass), most of which is present as arsenobetaine, an organic form of arsenic that has never been found in seawater. We report a method based on ion-exchange preconcentration and HPLC/mass spectrometry to measure arsenobetaine in seawater, and apply the method to samples of seawater collected at various depths from seven sites in the North Atlantic. Arsenobetaine was detected in most samples at levels ranging from 0.5 to 10 ng As kg-1, and was found at depths down to 4900 m. Furthermore, we report the presence of 15 additional organoarsenicals in seawater, 14 of which had never been detected in marine waters. The arsenobetaine depth profile was related, albeit weakly, to that of chlorophyll; this relationship probably reflects arsenobetaine's release to water from marine animals associated with the euphotic zone rather than its direct biosynthesis by primary producers. Future application of the new method for seawater analysis will shed new light on the biogeochemical cycle of marine arsenic.