The anthelmintic effect of the phloroglucinol derivative desaspidin in the treatment of fish tapeworm (Diphyllobothrium latum) infection was studied. The drug was used in doses of 200 and 400 mg. Thirty grams of castor oil emulsion was given as a purgative. The 200 mg dose of desaspidin was effective. A total of 2,368 persons received treatment. A significant side effect was observed in only one subject.