Experiments were conducted on the toxicity to fingerling brown trout of DDT when applied in an aqueous suspension, as an emulsion, as a solution in which the DDT is dissolved in a petroleum oil such as kerosene, and finally as as dust. The experiments were conducted at the Cornell University Experimental Hatchery in 15-liter aquaria containing 10 liters of water placed in a trough of running water to maintain a nearly constant temperature. Ten brown trout were used in each aquarium in each experiment. Measure of toxicity was taken as time required for a 50-percent mortality.