A method for the selective determination of trimethylamine (TMA) in aqueous matrices by liquid chromatography is reported. The proposed procedure is based on the derivatization of the analyte with 9-fluorenylmethyl chloroformate (FMOC) in a precolumn (Hypersil C18, 30 microm, 20 mm x 2.1 mm i.d.) connected on-line to the analytical column (LiChrosphere 100 RP18, 5 microm, 125 mm x 4 mm i.d.). Gradient elution was performed with a mixture of acetonitrile-water-0.05 M borate buffer (pH 9.0). The method has been applied to the direct determination of TMA in water within the 0.25-10.0 microg/ml concentration interval, and can also be adapted to the determination of TMA over the range 0.05-1.0 microg/ml by incorporating a preconcentration stage with C18 solid-phase extraction (SPE) cartridges. Good linearity, reproducibility and accuracy was achieved within the tested concentration intervals. The limits of detection at 262 nm were 50 and 5 ng/ml for the direct method and for the method involving preconcentration, respectively. The proposed conditions allowed the selective determination of TMA in the presence of other primary and secondary short-chain aliphatic amines. The utility of the described procedure has been tested by determining TMA in different water samples.