Triclosan in the waste, river and sea water samples collected in Hong Kong was analyzed by using gas chromatography-ion trap mass spectrometry method. (13)C(12)-triclosan was used as internal standard for the quantitative analysis. Water samples were prepared and cleaned-up by using a C(18) solid-phase extraction cartridge. The recoveries of triclosan in spiked coastal water at three different concentrations ranged from 83 to 110%. The method detection limit was 0.25 ng/L for triclosan in 1-L water and the relative standard deviations and relative error were less than 11.0 and 12.3%, respectively (n=3). The method was successfully applied to analyze water samples collected from rivers, coastal water bodies and wastewater treatment plants at ng/L levels.