Gnathostomiasis is primarily a disease of the skin characterized as creeping eruption or mobile erythema. However, larval Gnathostoma sometimes migrate into an unexpected site to elicit serious illness. Here we describe a case of colonic ileus caused by Gnathostoma doloresi. The patient was a 57-year-old man living in Miyazaki Prefecture, Japan, which is known as an area endemic for this parasite. One week after having eaten a few slices of the flesh of a snake (Agkistrodon halys), he developed severe abdominal pain. An abdominal radiograph revealed multiple gas-fluid levels with a distended bowel of an inverted U shape. A barium enema revealed a tumor in the ascending colon near the hepatic flexure that was surgically removed by simple colonic resection. An oblique section of a parasite surrounded by massive infiltration of eosinophils was found by postoperative histopathologic examination. The entire body of the advanced third-stage larva of G. doloresi was dissected from a specimen-embedded paraffin block.