By means of an experimental arrangement, by which only gaseous, volatile substances were detected, the phytonicide of Robinia pseudacacia was isolated from the plant infusion and concentrated by zonal melting and normal freezing. By its 2,4-dinitrophenylhydrazone, its UV and IR absorption spectra, and also by synthesis, it was identified as [DELTA]2 hexenal. Pharmacologically it is only slightly toxic but shows phytonicidal, fungicidal and protisticidal properties. In addition it was recognized as a phytonicide for oak, alder, lupine, black currant, bilberry, mountain cranberry, and also of privet and of grasses. In tests in the open it was demonstrated that [DELTA]2-hexenal is given off also by undisturbed plants.