Laboratory experiments were undertaken to investigate the allelopathic activity of Mexican sunflower in soil as well as to determine the effect of several soil factors on activity. There was a decrease in shoot and root growth of the test plant species when grown in soil previously planted with Mexican sunflower. However, seed germination was not affected. Soil-water separated from the same soil also inhibited shoot and root growth. A water extract from Mexican sunflower leaves applied to soil also reduced shoot and root growth, but the degree of inhibition varied among test plant species. Incorporation of dry Mexican sunflower leaves into soil at the rate of 1 and 2% (w/w) inhibited the growth of rice seedlings. The phytotoxic activity of incorporated leaf residue diminished 4 wk after treatment of soil. The inhibitory effect of a leaf water extract in nonautoclaved soil was less than in autoclaved soil and the phytotoxic effect to rice seedling grown in autoclaved soil was less than in autoclaved sand. It is suggested that the decrease in allelopathic activity of Mexican sunflower extract and its residue in soil is due to the action of soil microorganisms and the soil adsorption of the active components.