1. Clinical symptoms and oral treatment with metronidazole were studied in 103 patients with pure infections by Blastocystis hominis. 2. The results showed that excessive flatulence is the chief gastrointestinal symptom associated occasionally with diarrhoea and abdominal cramps. All the patients showed good responses with treatment of metronidazole and 74 patients whose stools were reexamined 1-2 months after the treatment demonstrated no signs of infections. 3. It is concluded that B. hominis is a pathogenic intestinal parasite and the infection could be eradicated successfully by oral metronidazole.