The allelopathic effects of juglone and leaf extracts of walnut (Juglans regia L.) on seed germination and seedling growth of 11 species were investigated. Seed germination was less affected than root and shoot growth in all species. Both seed germination and seedling growth of tomato, cucumber, garden cress and alfalfa were inhibited strongly by the treatments. However, seed germination of wheat, barley, corn, watermelon, radish and bean was not affected but their seedling growth was inhibited slightly. In muskmelon, interestingly, seedling growth was increased by both walnut leaf extracts and juglone. Positive correlation was found between the effects of juglone and the extracts.