A high-performance liquid chromatographic (HPLC) method for phenoxy acid herbicides using precolumn derivatization with 9-anthryldiazomethane (ADAM) is presented. The phenoxy acid herbicides investigated were (2,4-dichlorophenoxy)acetic acid, (4-chloro-2-methylphenoxy)acetic acid, 2-(4-chloro-2-methylphenoxy)propionic acid and (4-chloro-2-methylphenoxy)butyric acid. These herbicides reacted with ADAM under mild conditions and were converted into the corresponding fluorescent derivatives. The ADAM derivatives were separated by reversed-phase HPLC and determined using a fluorescence detector. The detection limits were about 500 pg per injection. For the application of ADAM to the determination of these herbicides in ground waters, the recoveries were more than 93% and the average relative standard deviation was 6.0% at 0.5 microgram/l. The procedure is useful as a screening method for phenoxy acid herbicides in ground water samples.