Emamectin benzoate is a very effective agricultural insecticide applied to various crops at very low rates. A method to analyze emamectin benzoate in seawater and freshwater was developed. The method yields good recoveries with limits of quantitation of 20 pg/mL for freshwater and 24 pg/mL for seawater (signal-to-noise ratio [S/N] ≥ 10). The limit of detection is 10 pg/mL (S/N ≥ 3). Emamectin benzoateis concentrated from a 50 mL sample on a C8 solid-phase extraction cartridge and eluted with 1% ammonium acetate in methanol. The eluate is evaporated to 1 mL, diluted with 1 % ammonium acetate in water, and partitioned into ethyl acetate. The ethyl acetate extract is concentrated to about 0.5 mL and diluted to about 1 mL with acetonitrile. 1-Methylimidazole is added to the diluted sample, and then trifluoroacetic anhydride is added to form a fluorescent derivative. The derivatized sample is injected onto a reversed-phase liquid chromatographic system equipped with a fluorescence detector. Overall method recoveries for concentrations ranging from 20 to 1010 pg/mL were 84 ± 9.5% for tap water and 89 ± 11% for seawater. These results are equivalent to recoveries previously obtained with a more elaborate method (90 ± 8.8% for freshwater). Controls were free of interferences or coeluting impurities. Fluorescence detection provides high sensitivity and selectivity, enabling trace-level analysis.