The allelopathic potential of cucumber (Cucumis sativus L.) plants after their crop harvest was investigated. Aqueous methanol extracts inhibited the growth of shoots of cress (Lepidium sativum L.), alfalfa (Medicago sativa L.) and barnyardgrass (Echinochloa crus-galli (L.) Beauv). A potent growth inhibitory substance in the extract was isolated by chromatographic separation of silica gel and Sephadex columns, Sep-Pack cartridge and HPLC. At concentration of 3 μg mL⁻¹, this substance inhibited cress shoot growth by 23% of shoot growth of control plants. These results suggest that cucumber plants may contain at least one growth inhibitory substance, which may be released into the environment, either as exudates from living plant tissues or leachates from residues of the plants, and may act as allelochemicals to neighboring plants.