Therapeutic activity of metronidazole against Balantidium coli was evaluated by using two drug regimens in 20 human cases of balantidiasis. All patients also harbored Trichuris trichiura. Children received a total amount of drug that varied from 2.5 g during 5 days to 7.5 g in a period of 10 days. Adults were given 5 g in 5 days or 12.5 g during a period of 10 days. In all patients B. coli disappeared after the 4th day of treatment. Frequent posttreatment stool examinations remained negative. All patients tolerated the drug well and there were no side effects.