A simple, economic, sensitive and rapid method for the determination of the pesticide diquat was described. This new method was based on the coupling of FIA methodology and direct chemiluminescent detection; to the authors knowledge this approach had not been used up to now with this pesticide. It was based on its oxidation with ferricyanide in alkaline medium; significant improvements in the analytical signal were achieved by using high temperatures and quinine as sensitizer. Its high throughput (144 h -1 ), together with its low limit of detection (2 ng mL -1 ), achieved without need of preconcentration steps, permitted the reliable quantification of diquat over the linear range of (0.01-0.6) µg mL -1 in samples from different origins (river, tap, mineral and ground waters), even in the presence of a 40-fold concentration of paraquat, pesticide commonly present in the commercial formulations of diquat.