THE movement of unchanged DDT from treated crops into the atmosphere has been suggested to contribute to the widespread contamination of the environment by this substance1. In field conditions, 50% of DDT applied to the surface of soil has been found to disappear in 16–20 weeks (ref. 2) and 60% of DDT applied to an apple orchard could not be accounted for3. Ward and Burt4 attributed losses of DDT from glass plates and leaf surfaces to evaporation, but little other work seems to have been published on this question.