The main metabolite of the herbicide pyridate is 3-phenyl-4-hydroxy-6-chlorpyridazine (CL9673). A high-performance liquid chromatographic method with amperometric detection is described for determining CL9673 at residue levels in water samples. Sample preconcentration is performed by passage through a C18 extraction cartridge. A recovery study using tap water samples spiked with CL9673 at a concentration of 0.1 micrograms/l showed a recovery of 84.8% (coefficient of variation 6.2%). The method is suitable for the determination of CL9673 in drinking and groundwater.