The main symptoms of infection with Giardia intestinalis in 33 Sudanese adults were abdominal pain, flatulence and diarrhoea. Other symptoms were offensive stools, loss of weight, milk intolerance, mucoid stools, nausea and vomiting. Tinidazole given orally as 150 mg. twice daily dose for seven days or as a single 1,000 mg. dose, eradicated the infection in 32 out of the 33 patients. This parasitological cure was obtained in all 21 patients who received the seven day course and in 11 out of 12 patients who were given the single dose. All symptoms disappeared in 16 out of the 32 patients and in the remaining half improvement was marked particularly as regards the three main symptoms of the disease. The single dose gave no side effects but nausea, headache, skin rash and worsening of abdominal pain and diarrhoea occurred in some patients who received the seven day treatment course.