The rarity of reported cases of balantidiasis in the medical literature makes any new focus of infection worthy of note. Therefore when an active infection with Balantidium coli was found in a patient at the South Carolina State Hospital, an investigation was instituted to determine the prevalence of the parasite in a selected group of patients. The patients examined were those suffering from diarrhea and those with a mental condition which tolerated untidiness, as it was felt that if additional infections were present they were most likely to be found among these patients. Usually two fecal specimens from each patient were examined, but the stools of patients found to be infected with Balantidium coli were examined repeatedly. A simple saline smear preparation was used. No technics of concentration of parasites were employed. A total of seven infections were found among the 142 patients examined. This represents an incidence of 4.93