Toxicity of 3,6–endoxohexahydrophthalic acid (endothal) was measured by the inhibition of growth of germinating flax roots, compared to known standards. In the field, endothal applied at 0.3 to 10.0 ppm was not detectable after an average of 2½ days, and a maximum of 4 days. In aquaria, endothal at these rates was detectable much longer than this, and much longer when the aquaria contained only water than when they also contained mud and plant debris.