The Autors study the therapeutic efficacy of Tinidazole, a new nitrimidazole derivative, in the treatment of thirty patients with giardiasis. They employed the daily dosage of 300 mg for twenty patients and 150 mg for ten children, respectively with ten years of age or and less during five consecutive days. The diagnosis and the parasitological evaluation of the results of treatment were accomplished by stool examinations using zinc sulphate centrifugal-flotation technique of Faust et al. and sedimentatian-concentration technique of Hoffman, Pons and Janer. The control started one week after the treatment and consisted of three stool examinations, at one week intervals. The Authors achieved 96.6 per cent of parasitological cures. They stressed the excellent tolerance of the drug ,even by patients with serious fundamental diseases or by children. No side effects were registered. They conclude that Tinidazole is an excellent drug against Giardia lamblia infections.