Treating 30 adult patients bearers of Giardia lamblia infection with tinidazole at a dosage of 150 mg. twice a day for seven consecutive days, the authors have obtained a cure rate of 93.3%. The tolerance to the drug was, generally, very satisfactory. In view of the results obtained, the authors pointed out that there is no question about being the drug used. - a new antipasasitic which is under many therapeutic trials at present - a most useful tool in treatment of infections caused by Giardia lamblia. After this investigation was carried out, the authors also considered it timely to establish new dosage schedules in order to obtain even better results.