Ailanthus altissima (Mill.) Swingle contains one or more phytotoxic compounds in roots and leaves. Activity is higher in roots, where it occurs primarily in the bark. Powdered root bark and leaflets strongly inhibited growth of garden cress (Lepidium sativum L.) when mixed with soil in Petri dishes (ID50 values=0.03 g root bark, 0.6 g leaflet/dish). The toxic material was readily extracted by methanol but not dichloromethane. Pieces of root bark mixed with soil at 2, 1, and 0.5 g/pot reduced cress biomass in the greenhouse, whereas methanol-extracted root bark did not. The inhibitory effect ofAilanthus tissues in soil was short-lived (≤4 weeks in pots in greenhouse, ≤3 days in Petri dishes in laboratory). Inhibition by root bark was sometimes superseded by stimulation. FreshAilanthus root segments placed in or on soil reduced growth of nearby cress seedlings. Fine roots were more inhibitory than coarse, and inhibition became more pronounced with increased time of soil exposure to roots. Soil collected nearAilanthus roots in the field supported reduced radicle growth of cress compared to control soil. In contrast, stemflow fromAilanthus trees stimulated cress growth. The results suggest allelopathy caused by toxin exudation from roots may contribute to the aggressiveness and persistence ofAilanthus in certain habitats.