A 42-year-old Thai man from central Thailand came to Ramathibodi Hospital with the complaint of abdominal pain. Physical examination revealed localized tenderness in the right lower quadrant. The clinical impression was acute appendicitis and an operation was performed, revealing a large mass in the cecum. A right hemicolectomy was done for what was thought to be a carcinoma of the colon. The resected colon showed subserosal thickening on the antimesenteric side of the ascending colon with a constricted area at the middle portion. An immature adult Gnathostoma spinigerum was identified in the thickened subserosa of the cecum. Microscopic examination of the involved bowel showed a heavy infiltration of eosinophils, fibroblasts and histiocytes, and mild to moderate edema. The findings were consistent with eosinophilic granuloma of the gastrointestinal tract, and the presence of the parasite in the affected bowel suggested that it was the etiologic agent.