A 65-year-old Japanese female was admitted to our hospital because of right lower abdominal pain. Laboratory and roentgenological tests revealed no abnormalities. Repeated stool examinations revealed no ovum or parasite. However, colonoscopic examination demonstrated the presence of a parasite on the edematous mucosa of cecum. The adult male whipworm (Trichuris trichiura) was then removed endoscopically. Her symptoms disappeared quickly, and no additional parasite was found in the feces during or after the mebendazole treatment. Repeated stool examinations failed to show any ova, because this patient was infected by a single male parasite. Colonoscopic study should be considered a useful tool for the diagnosis of T. trichiura infection, even if no ovum is found in stool examination.