Anisakiasis of the duodenum has been observed infrequently. Moreover, there has been no report of concurrent occurrence of small bowel obstruction and fistula formation for duodenal anisakiasis. We describe an interesting and instructive case of a patient with duodenal anisakiasis consisting of small bowel obstruction and fistula formation. The differential diagnosis was performed by observing the morphological patterns more closely with the use of CT imaging, MR imaging and an upper gastrointestinal (UGI) series.