Juglone concentration was measured in soils collected under a 14-year-old black walnut (Juglans nigra L.) plantation containing plots of pure walnut and of walnut mixed with either autumn-olive (Elaeagnus umbellata Thunb.) or European black alder (Alnus glutinosa (L.) Gaertn.). Juglone concentrations declined with soil depth and distance from the walnut tree. Concentrations in autumn-olive-walnut plots were statistically lower than concentrations in European black alder-walnut or pure walnut plots. The concentration of juglone in soil in European black alder-walnut plots was apparently sufficient to cause the onset of black alder mortality.