A suspension of DDT in Lake Erie water was made by adding 1 ml. of a 1% soln. of DDT in acetone to 249 ml. of water for initial cones. In all but one instance, 50% of Daphnia magna were immobilized by cones, of over 1 part per billion in 32 hrs. or less. Animals placed in lesser cones. survived as long as controls in Lake Erie water alone.