The susceptibility of wild mallard ducklings to the delayed neurotoxic effect of the neurotoxic organophosphorus insecticides cyanofenphos and leptophos was evaluated following a daily dosing regimen. Ducklings were treated daily with either cyanofenphos or with leptophos at different dose levels for 90 days, or until they died, or became paralyzed. A control group of ducklings given corn oil at 1 ml/kg daily for 90 days was used for comparison. All treated birds were observed daily for any clinical signs of neurotoxicity during the course of this study. All of the surviving ducklings that were treated with cyanofenphos at 4 mg/kg/day or leptophos at 10 mg/kg/day developed clinical signs of delayed neurotoxicity after 7 to 11 weeks of intoxication. Symptoms included leg weakness, ataxia, severe ataxia and paralysis. The observed clinical signs were confirmed by histological changes found in the spinal cords of the treated birds. These changes were of the type associated with organophosphorus-induced delayed neuropathy (OPIDN). These results demonstrate that wild mallard ducklings are susceptible to OPIDN and this avian species can be used in screening organophosphorus compounds for such effect.