We have reported a case of diarrhea caused by Blastocystis hominis, an intestinal protozoan parasite of man. The organism is present in small numbers in up to one fifth of stool samples in hospitalized patients, but is associated with diarrhea in only heavily infested patients. Typical symptoms include diarrhea, crampy abdominal pain, nausea, vomiting, low-grade fever, gas, malaise, and chills. Fecal leukocytes are occasionally seen. The pathophysiologic mechanism of the diarrhea is not clear. Not all patients having large parasite burdens are symptomatic. Metronidazole, 1 to 2 gm/day orally in divided doses, is the treatment of choice.