The toxicity of the herbicide Agroxone 5, a commercial formulation of the iso-octyl ester of 2,4-dichlorophenoxyacetic acid (2,4-D), in the adult crested newt (Triturus cristatus carnifex) was tested after percutaneous exposure. The compound was lethal to all the animals within 72 hr at concentrations of 100, 125, and 150 ppm. Males appeared to be more susceptible than females to the toxic effects of 2,4-D, since the LT50 values at 75 ppm were 102 and 132 hr for male and female newts, respectively. At 50 ppm, all the males died within 31 days of exposure, whereas the females were still alive at the end of the 3-month experiment. Only one male died at 25 ppm after 21 days. Vacuolar degeneration of liver parenchyma and necrosis of kidney tubules were found at lethal concentrations of 2,4-D. The severity of the lesions was related to the duration of exposure more than to the concentration of the herbicide.