The case reported here concerns an alcoholic pork-butcher who presented with severe colitis with peritonitis, caused by the only ciliate protozoan capable of infecting humans, Balantidium coli. This parasite is common in a variety of domestic and wild mammals, mainly pigs; however, its prevalence rate in humans is very low--particularly in industrialised, northern countries, including France. The infection is most frequently acquired by ingesting food or water contaminated by pig faeces, and it may be asymptomatic or may cause acute diarrhoea. Specific antibiotic treatment is efficacious, and it is important to consider the risk of this parasitic disease in susceptible patients presenting with bloody diarrhoea.