A dead whooper swan was found in an area of cropland near a stream and was submitted to the National Veterinary Research and Quarantine Service (NVRQS) in Korea. The affected animal was in relatively good condition. Grossly, the heart was enlarged and had pale and dark red stripes. A white, elongate parasite was seen on the cut surface of the heart. Histopathologically, severe lymphohistiocytic inflammation, myocardial necrosis, many adult heart worms and microfilariae were observed in the myocardium. Hemorrhage, lymphocytic inflammation, mineralization, and myocardial degeneration were also seen around the adult worms. No bacteria or viruses were isolated from the affected bird. The pathological findings indicate that the whooper swan was infected with nematodes, presumably Sarconema eurycerca, resulting in non-suppurative myocarditis.