Over recent decades there have been numerous reports of the occurrence of trimethylamine-N-oxide (TMAO) in marine organisms, including algae, zooplankton and fish, where it is proposed to function as an osmolyte and may be involved in a number of other physiological roles. While it may seem reasonable to assume that TMAO should also be present in seawater, to date its occurrence has not been reported. Here we report the first quantitative measurements of TMAO concentration and distribution in seawater. Results are also compared to the structurally and biochemically analogous sulphur compound, dimethylsulphoxide (DMSO) and its precursor dimethylsulphide (DMS). Studies were conducted in the coastal waters off the Antarctic Peninsula between January and February 1999. Concentrations of dissolved TMAO ranged from below the analytical detection limit (1.65 nmol dm-3) to 76.9 nmol dm-3 in the upper water column (to 100 m), with a mean of 15.2 nmol dm-3. These concentrations are comparable to those of DMSO, (mean 8.7 nmol dm-3) and DMS (mean 3.0 nmol dm-3), and are highly comparable to those of other nitrogen species, such as the methylamines (MAs). In vertical profiles, greatest concentrations of TMAO were observed in surface waters where they exceeded those of all other methylamine analytes.