Abstract: A method was proposed for the determination of metalaxyl residues in 4 aquatic products（fish, shrimp, crab and shellfish）by isotope dilution-liquid chromatography-tandem mass spectrometry. The sample(5±0.05) g was taken, and 10 ng of internal standard metalaxyl-d₆ was added, settling for 10 min. Then the mixture was vortex mixed and centrifuged in 10 mL of acetonitrile solution containing 1% (volume fraction) acetic acid. The supernatant was blown to less than 0.5 mL by nitrogen at 45 ℃, and then 7.5 mL of 20% (volume fraction, the same below) acetonitrile solution was added to mix well. 7.5 mL of n-hexane was used to defat, and Agela Cleanert S C₁₈ solid phase extraction column was used for enrichment and purification. The obtained solution was blown to dryness by nitrogen at 45 ℃, and the residue was redissolved in 1 mL of 20% acetonitrile solution. After mixed well, the solution was passed through 0.22 μm needle aqueous nylon filter membrane and the filtrate was collected into the sample vial. SHISEIDO CAPCELL PAK C₁₈ column was used as stationary phase, and gradient elution was performed with a mixture of 0.1% (volume fraction) formic acid solution and acetonitrile at different volume ratios as mobile phase. The internal standard method was used for quantitative analysis. As shown by the results, linear range of the standard curve of metalaxyl was 1.0-100.0 μg·L^-1, with detection limit (3S/N) of 0.5 μg·kg^-1. Using different varieties of aquatic products as blank matrices, the recovery ranged from 93.8% to 105% at 3 concentration levels of 1.0, 2.0, 10.0 μg·kg^-1, with RSDs (n=6) of the determined values less than 13%.