Thiabendazole, in the dosage of 2 g, approximately 40 mg/kg, as a single dose for 1 day in adult patients, has a moderate anthelmintic activity against both Ancylostoma duodenale and Necator americanus as judged by egg-count reduction. Twenty-five mgAg on 2 consecutive days was less effective. However, the real efficacy of this drug against hookworm is not so great as the degree of egg reduction suggests, for subsequent therapy with bephenium hydroxynaphthoate removed a large number of worms. This compound also has anthelmintic efficacy for Ascaris lumbricoides infection in man, a dose of 25 mg/kg on 2 successive days reduced the egg count 75%. No activity against Trichuris trichiura was found. Although transient side reactions of dizziness and nausea following administration of the drug are not serious, the incidence of these reactions is high.