A 28-year-old man was hospitalized for a dysenteric syndrome that had developed during the previous days. Physical examination revealed abdominal pains, fever, vomiting and more than ten liquid stools per day. Fresh stool examination showed numerous mobile ciliated trophozoites of Balantidium coli. The patient reported having been on a hike the previous weekend during which he had drunk water through a hydration pouch bladder. Complete resolution was observed after intravenous rehydration and ten days of oral treatment with metronidazole (Flagyl®). Balantidium coli is the largest ciliate protozoan able to infect humans. This parasite is common in pigs and has a worldwide distribution. Human infections, a rare event in industrialised countries, are usually acquired by ingestion of food or water contaminated by mammal faeces. Human B. coli infections are easily treated but may be severe and even fatal if neglected.