Tests were conducted to determine the effects of nine insecticides and five molluscicides upon blood-sucking aquatic leeches, Hirudo nipponia, from Japan and Korea. Under test conditions similar to those followed in WHO procedures for determining susceptibility or resistance of mosquito larvae to insecticides, leeches were highly tolerant to DDT (LC50 > 100 ppm) and to several other insecticides and molluscicides. Chlordane (LC50 1–2.3) was the most effective of the insecticides tested, and diazinon (1.5–7) was second in effectiveness. Dinex (0.1–0.3) and an experimental material, I.C.I. 24223 (0.3–0.4), were the most toxic of the molluscicides tested. Levels of tolerance to both insecticides and molluscicides were such that it would be impractical to use the substances tested to control leeches in the field unless the killing of fish and other animals in treated waters were unobjectionable.