BACKGROUND: Diphyllobothriasis is estimated to afflict 10–20 million people worldwide; however, this is the first case reported in a United States military aviator. Among the largest parasites of humans, the “fish tapeworm” grows from 2–15 m in length, can live >20 yr in the intestines, and is contracted through consumption of uncooked, unfrozen freshwater or anadromous fish species.CASE REPORT: A 32-yr-old male F-22 pilot presented with mild stomach cramping, bloating, nausea, and intermittent loose stools. Symptoms were relieved with bismuth subsalicylate until several days later when the patient, during otherwise normal bowel movements, extracted multiple broken segments of tapeworm. Although physically asymptomatic, he was psychologically disturbed. Based on the large number of ova with characteristic shape, size, color, and operculum, coupled with the flattened body, yellowish coloration, and rectangular proglottids with centrally located “rosette” uteri, he was diagnosed with diphyllobothriasis (likely D. latum or D. nihonkaiense). Successful treatment with a single oral dose of praziquantel (>10 mg · kg⁻¹) was confirmed by negative stool examination over 60 d posttreatment. He likely contracted the parasite from ingesting salmon sushi or sashimi while previously stationed in Japan.DISCUSSION: Despite only mild physical symptoms, the pilot’s psychological distress and distraction from knowing about the meters-long tapeworm was significant. Prompt treatment was paramount to resumption of military operations. Aviators should be educated and encouraged to eat only well-cooked or previously frozen fish, especially when indulging in cultural cuisine.Kasteler SD. Diphyllobothriasis in a U.S. military aviator. Aerosp Med Hum Perform. 2018; 89(5):473–477.