Seasonality of the nematode Gnathostoma turgidum in Virginia opossums (Didelphis virginiana) in the wild has been reported; however, the mechanisms involved in deworming are unknown. We monitored the parasitologic and biologic changes in four Virginia opossums naturally infected with G. turgidum by coproparasitologic examination and abdominal ultrasonography. Eggs became detectable in the feces of opossums in May, peaked in July and August, and suddenly decreased in October. Adults of G. turgidum were expelled in the feces mainly in September. Ultrasonography of the liver showed slight damage during May. Lesions in the stomach appeared in April and persisted until September. The abnormalities of the liver and stomach were resolved in November. These data suggest that G. turgidum is likely expelled as a result of host immunologic mechanisms, although termination of a natural life span cannot be definitively excluded.