A gas chromatography-ion trap mass spectrometry method was developed and optimized for the analysis of triclosan in water. Tandem mass spectrometry, along with an isotope dilution internal standard method, was used for the quantitative analysis of triclosan in water at low ng l(-1) levels. The efficiencies obtained from liquid-liquid extraction and solid-phase extraction were compared. Average recoveries by the SPE pre-concentration using a C(18) cartridge were determined as 84-90%. The limit of detection was 2 ng l(-1) for triclosan in water. The accuracy represented by relative analytical errors was -16% to -10%, and the precision by relative standard deviations was 3-15% (n=4). The method was successfully applied to analyze triclosan at concentrations between 4.1 ng l(-1) and 117 ng l(-1) in environmental water samples collected from rivers and coastal water in Hong Kong.