The toxicity of organophosphate arises from disruption of the nervous system due to the inhibition of cholinesterase enzymes, leading to death. Six dead mallards were found at Ansung where is one of the most popular wintering sites for migratory birds in Korea, and requested for diagnosis to Gyeonggi Veterinary Service on January of 2007. Some examinations including polymerase chain reaction (PCR) could not find any evidence of specific disease condition. However, the contents of gastrointestinal tracts of the birds contained residues of monocrotophos ranged from 31.3ppm to 294.3ppm by gas chromatography and mass spectrometry. It can be supposed that monocroptophos was responsible for the death of mallards by this results.