Praziquantel (Droncit) is a new anthelminthic agent with a strikingly wide therapeutic index. The efficiency of the drug against Diphyllobothrium latum was tested in vivo and in vitro. With a drug dose of 50 mg/kg, artificially infected hamsters were completely cured. The threshold value of the active dose was between 2·5 and 10·0 mg/kg. Clinical trials with Praziquantel against human diphyllobothriasis show great promise, all of ten patients treated with 25·0 mg/kg of the drug being cured. Praziquantel had little effect on larval or adult diphyllobothriids in vitro.