Fenbendazole (Hoe 881) 5-(phenylthio)-2-benzimidazolecarbaminicacidmethylester in doses of 1-0 g and 1-5 g per person were effective against Ascaris and gave substantial egg count reductions against hookworm (mainly Necator americanus). The 1-5 g dose gave good results against Trichuris. In a trial with a suspension of Hoe 881 with a high specific surface of 25 m2/g there was no advantage over tablets with a specific surface of approximately 8 m2/g. Two doses of 500 mg given with an interval of 24 hours were no more effective than one dose of 1-0 g. In a comparative trial using Hoe 881 1-5 g, Pyrantel 10 mg/kg and placebo respectively Hoe 881 showed equal potency against hookworms and Acsaris as Pyrantel and good effectiveness against Trichuris. Pyrantel showed only moderate activity against Trichuris in this trial.