Balantidium coli is the only parasitic ciliate of man. It is a flattened oval organism covered with cilia, and a gullet at the anterior end. It is infrequently pathogenic for man, although epidemic buds in tropical zones have been described. The infection fundamentally affects the colon and causes variable clinic pictures, from asymptomatic to serious dysenteric forms. We present a case of parasitologically diagnosed as causes of diarrhea in a patient with non-Hodgkin's lymphoma from Turkey. In order to find out the causative etiologic agent of diarrhea, stool samples were examined by native, lugol and flotation methods and we detected moving trophozoites, which were approximately 60 microm long and 35 microm wide. These bodies were diagnosed as Balantidium coli. This case underlines that Balantidium coli should also be considered as a possible pathogen in immunocompromised patients with diarrhea.