Juglone is a natural constituent of walnut with phytotoxic and allelopathic properties. Its herbicidal effects on the weed species wild mustard (Sinapis arvensis L.), creeping thistle (Cirsium arvense L.), field poppy (Papaver rhoeas L.) and henbit (Lamium amplexicaule L.) were investigated using wheat (Triticum vulgare Vill. cv. Gerek 79) and barley (Hordeum vulgare L. cv. Kislik) varieties as control plants. A juglone concentration of 5.74 mM had a lethal effect on field poppy. Juglone concentrations of 1.15 and 2.30 mM decreased the elongation and fresh weight of the weeds, but not wheat and barley The elongation of the shoot was more significantly decreased than that of the root in all weeds. Chlorophyll content of the weeds was also decreased by juglone. The study reveals that juglone is a potent inhibitor of growth of the weeds and therefore it can be evaluated as a herbicide for future weed management strategies.